jrsonnet-fmt: --use-tabs now defaults to disabled

[JaySon-Huang]

Problem Summary

In cmds/jrsonnet-fmt/src/main.rs, the --use-tabs flag was declared as:

/// Force hard tab for indentation
#[arg(long, default_value = "true")]
use_tabs: bool,

The help text ("Force hard tab") implies an opt-in boolean flag that is off by default, but the default_value = "true" made it always true:

• not passing the flag ==> true
• passing the flag ==> true
• --use-tabs=false ==> clap error ("unexpected value")

So the formatter always emitted Tab indentation, and the --indent <N> option had no visible effect.

Fix

Remove the default_value so the flag is a plain opt-in

[JaySon-Huang]

/cc @CertainLach