feat(cli): add --multi-output option for separate grammar compilation

[knoan]

Summary

Add --multi-output <dir> CLI option that compiles multiple input grammars to separate output files instead of merging them into a single parser.

Changes

• Add --multi-output <dir> option to CLI with .conflicts("output")
• Add validation in opts.js (cannot use with stdin)
• Implement multi-output branch in run() method
• Add 5 tests for the new feature
• Update documentation and changelog

Usage

peggy --multi-output ./out/ grammar1.peggy grammar2.peggy
# produces: ./out/grammar1.js, ./out/grammar2.js

peggy --multi-output ./out/ --dts --source-map -- a.peggy b.peggy
# produces: ./out/a.js, ./out/a.d.ts, ./out/a.js.map, ./out/b.js, ./out/b.d.ts, ./out/b.js.map

Test plan

• Unit tests added (5 tests)
• Manual testing completed
• Lint passes
• All 53 CLI tests pass

Closes #650

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.48%. Comparing base (f4e5c5a) to head (ddcb829).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #651   +/-   ##
=======================================
  Coverage   99.48%   99.48%           
=======================================
  Files          34       34           
  Lines        4283     4313   +30     
=======================================
+ Hits         4261     4291   +30     
  Misses         22       22           

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[hildjj]

Will do a full review tomorrow, probably. In the meantime, it should be relatively easy to add coverage for the two lines that codecov flagged.

[knoan]

Will do a full review tomorrow, probably. In the meantime, it should be relatively easy to add coverage for the two lines that codecov flagged.

Done

[hildjj]

LGTM. Fantastic patch, thank you very much.

[hildjj]

To be release in Peggy 5.1, hopefully this week.