New feature: break/continue in loops

[cknitt]

Summary

Add support for break and continue in ReScript while and for loops.

This change makes break and continue real keywords, supports using them from within a switch nested inside a loop, and emits native JS break / continue in the generated output.

Implementation

• Add break / continue as reserved syntax tokens in the ReScript parser.
• Add dedicated AST / typedtree / lambda nodes for loop control.
• Keep parsetree0 frozen and bridge the new nodes through reserved internal extensions in the v0 mappers.
• Typecheck break / continue as loop-only control flow and reject them in nested function bodies.
• Reuse warning 21 for break / continue in statement position, matching existing throw(...) behavior, with clearer wording for non-continuing control flow.
• Lower loop control into JS break / continue.
• Support breaking/continuing from a nested switch by generating loop labels when needed.
• Allocate loop labels lazily so only loops that actually need labeled control flow consume loop_<n> ids.

Notes

• break and continue are now reserved keywords, so existing identifier uses must be escaped or renamed.
• The backend only emits loop labels when required for correctness, which keeps the generated JS cleaner.
• String-switch test cases were added alongside integer-switch cases because they preserve a JS switch in the output and therefore exercise the labeled codegen path directly.

Tests

• Syntax tests for parsing and printing break / continue.
• Super-error tests for:
  • break / continue outside loops
  • nested-function rejection
  • reserved-keyword binding errors
  • warning 21 on unconditional break / continue statements with following dead code
• Build test coverage for lambda output and generated JS label emission.
• Runtime tests in tests/tests for:
  • while + break / continue
  • for + break / continue
  • nested switch targeting the enclosing loop
  • JS-switch cases that exercise labeled loop control

Verification

• make
• make lib
• node tests/build_tests/super_errors/input.js
• node tests/build_tests/loop_control/input.js
• node cli/rescript.js build in tests/tests
• npx mocha -t 10000 tests/tests/src/loop_control_test.mjs

[cknitt]

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commit: 484e6e7

[cknitt]

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Codex Review: Didn't find any major issues. 🚀

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