feat(prompt): support custom Copilot instructions file (#1467)

Add support for including custom Copilot instructions from
.github/copilot-instructions.md in the system prompt. Refactor prompt
resolution logic into a new prompt.lua module, and update function and
tool resolution to use this module. Also simplify diff hunk application
fallback logic.

Signed-off-by: Tomas Slusny <[email protected]>

Author: deathbeam

lua/CopilotChat/config.lua

@@ -14,7 +14,7 @@
 ---@field blend number?
 
 ---@class CopilotChat.config.Shared
----@field system_prompt nil|string|fun(source: CopilotChat.source):string
+---@field system_prompt nil|string
 ---@field model string?
 ---@field tools string|table<string>|nil
 ---@field resources string|table<string>|nil

lua/CopilotChat/config/mappings.lua

@@ -307,10 +307,10 @@ return {
       end
 
       local lines = {}
-      local config, prompt = copilot.resolve_prompt(message.content)
-      local system_prompt = config.system_prompt
 
       async.run(function()
+        local config, prompt = copilot.resolve_prompt(message.content)
+        local system_prompt = config.system_prompt
         local resolved_resources = copilot.resolve_functions(prompt, config)
         local selected_tools = copilot.resolve_tools(prompt, config)
         local selected_model = copilot.resolve_model(prompt, config)