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

[hemanth]

Description

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

1. Inline Syntax Highlighting

Uses the Acorn tokenizer (already in deps/) to colorize JavaScript input in real-time:

• Keywords (const, function, if, etc.) → Magenta
• Strings → Green
• Numbers → Yellow
• Booleans/null/undefined → Yellow
• Regular expressions → Red
• Comments → Gray

Highlighting is applied at display time only — repl.line remains plain text, so cursor positioning and evaluation are unaffected.

2. Auto-Indentation

Automatically inserts 2-space indentation on continuation lines based on brace/bracket/paren depth when entering multiline input.

3. Function Signature Hints

When the user types functionName(, the function's parameter list is displayed as a dimmed hint below the input line. Uses the V8 Inspector protocol with throwOnSideEffect: true for safety.

Usage

node --experimental-repl-enhancements

All features are off by default — nothing changes for users unless they explicitly pass the flag. Individual features can also be controlled via repl.start() options (e.g., syntaxHighlighting: false).

Motivation

This addresses the feature requests from #48164, which was closed as not planned. However, by gating everything behind an experimental flag with no behavioral changes to the default REPL, the risk profile is minimal. These are quality-of-life improvements that bring the Node.js REPL closer to modern shell experiences (Python's IPython, Ruby's Pry, etc.).

Refs: #48164

[nodejs-github-bot]

Review requested:

• @nodejs/config

[codecov]

Codecov Report

❌ Patch coverage is 31.09756% with 339 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.11%. Comparing base (14166a5) to head (14baff1).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/repl/utils.js	27.99%	318 Missing and 1 partial ⚠️
lib/repl.js	57.44%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64443      +/-   ##
==========================================
- Coverage   90.23%   90.11%   -0.12%     
==========================================
  Files         741      741              
  Lines      241375   241867     +492     
  Branches    45483    45509      +26     
==========================================
+ Hits       217800   217954     +154     
- Misses      15129    15471     +342     
+ Partials     8446     8442       -4     

Files with missing lines	Coverage Δ
src/node_options.cc	76.74% <100.00%> (+0.02%)	⬆️
src/node_options.h	97.53% <100.00%> (-0.49%)	⬇️
lib/repl.js	92.90% <57.44%> (-1.11%)	⬇️
lib/internal/repl/utils.js	73.11% <27.99%> (-23.28%)	⬇️

... and 29 files with indirect coverage changes

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