repl: add syntax highlighting, auto-indentation, and signature hints

Add experimental REPL enhancements behind the
--experimental-repl-enhancements flag:

1. Inline syntax highlighting using the Acorn tokenizer
   to colorize keywords (magenta), strings (green),
   numbers (yellow), booleans/null (yellow), regular
   expressions (red), and comments (gray).

2. Auto-indentation for multiline input based on
   brace/bracket/paren depth, using 2-space indentation.

3. Function signature hints that display parameter lists
   as dimmed text below the input when the user types a
   function call.

All features are off by default and require the CLI flag
to enable. Highlighting is applied at display time only
so cursor positioning and evaluation are unaffected.

Refs: #48164
Signed-off-by: hemanth <hemanth.hm@gmail.com>

Author: hemanth

doc/api/cli.md

@@ -1381,6 +1381,18 @@ added:
 
 Enable experimental support for the QUIC protocol.
 
+### `--experimental-repl-enhancements`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+Enable experimental REPL enhancements including inline syntax highlighting,
+auto-indentation, and function signature hints. See the
+[REPL][] documentation for details.
+
 ### `--experimental-sea-config`
 
 <!-- YAML
@@ -3829,6 +3841,7 @@ one is included in the list below.
 * `--experimental-package-map`
 * `--experimental-print-required-tla`
 * `--experimental-quic`
+* `--experimental-repl-enhancements`
 * `--experimental-require-module`
 * `--experimental-shadow-realm`
 * `--experimental-specifier-resolution`

doc/api/repl.md

@@ -27,7 +27,8 @@ result. Input and output may be from `stdin` and `stdout`, respectively, or may
 be connected to any Node.js [stream][].
 
 Instances of [`repl.REPLServer`][] support automatic completion of inputs,
-completion preview, simplistic Emacs-style line editing, multi-line inputs,
+completion preview, inline syntax highlighting, automatic indentation,
+function signature hints, simplistic Emacs-style line editing, multi-line inputs,
 [ZSH][]-like reverse-i-search, [ZSH][]-like substring-based history search,
 ANSI-styled output, saving and restoring current REPL session state, error
 recovery, and customizable evaluation functions. Terminals that do not support
@@ -439,6 +440,64 @@ function myWriter(output) {
 }
 ```
 
+### Syntax highlighting
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+These features require the `--experimental-repl-enhancements` flag.
+
+```bash
+node --experimental-repl-enhancements
+```
+
+When the flag is enabled and `useColors` is `true`, the REPL highlights
+JavaScript input using the following color scheme:
+
+* **Keywords** (`const`, `let`, `function`, `if`, `return`, etc.): Magenta
+* **Strings** (single-quoted, double-quoted, and template literals): Green
+* **Numbers**: Yellow
+* **Boolean literals** (`true`, `false`), `null`, `undefined`, `NaN`,
+  `Infinity`: Yellow
+* **Regular expressions**: Red
+* **Comments** (line and block): Gray
+
+Syntax highlighting is applied only to the display; the internal `line`
+property remains plain text. Highlighting can be disabled by passing
+`syntaxHighlighting: false` to [`repl.start()`][].
+
+### Auto-indentation
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+When the `--experimental-repl-enhancements` flag is enabled and entering
+multi-line input (e.g., a function body or an object literal), the REPL
+automatically indents continuation lines based on the current nesting
+depth of braces (`{}`), brackets (`[]`), and parentheses (`()`). The REPL
+uses 2-space indentation following the Node.js coding convention.
+
+### Function signature hints
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+When the `--experimental-repl-enhancements` flag is enabled and the user
+types a function name followed by `(`, the REPL displays the function's
+parameter list as a dimmed hint below the input line. This uses the V8
+inspector protocol to safely extract function signatures without side
+effects. Signature hints require the `preview` option to be enabled and
+the inspector to be available.
+
 ## Class: `REPLServer`
 
 <!-- YAML
@@ -791,6 +850,12 @@ changes:
     previews or not. **Default:** `true` with the default eval function and
     `false` in case a custom eval function is used. If `terminal` is falsy, then
     there are no previews and the value of `preview` has no effect.
+  * `syntaxHighlighting` {boolean} If `true`, enables inline syntax
+    highlighting of REPL input using ANSI color codes. Keywords are displayed
+    in magenta, strings in green, numbers in yellow, regular expressions in
+    red, and comments in gray. Requires `--experimental-repl-enhancements`,
+    `useColors` to be `true`, and `terminal` to be `true`.
+    **Default:** `true` when `--experimental-repl-enhancements` is set.
   * `handleError` {Function} This function customizes error handling in the REPL.
     It receives the thrown exception as its first argument and must return one
     of the following values synchronously: