Add Token.isLiteralIdentifier

[jparise]

Summary

Small readability change: names the fact that true, false, and nil are tokenized as identifiers rather than keywords, so rules don't each have to remember it.

Reasoning

Any rule that treats an identifier as a name referring to something has to exclude these three, and that check was written out by hand in a handful of places — including inside ParsingHelpers itself:

if prevToken.isIdentifier, !["true", "false", "nil"].contains(prevToken.string) {

This is easy to miss, which is the motivation: while writing preferLazyMap I did miss it, and the rule initially rejected any closure body containing a boolean literal. Putting the predicate next to isNumber and the other token checks makes it discoverable instead of rediscovered.

Adopted at the two sites where it's a drop-in replacement. The other sites match these literals as switch patterns (case .identifier("true"), .identifier("false"), ...), where spelling them out reads at least as clearly as a where clause would, so I left them alone. redundantType keeps its own true/false-only check on purpose — nil has no inferable type and shouldn't be folded in.

Full test suite passes.

[calda]

Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (adad8ac) to head (bd95b2b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2662      +/-   ##
===========================================
- Coverage    95.47%   95.46%   -0.01%     
===========================================
  Files          178      178              
  Lines        27186    27189       +3     
===========================================
+ Hits         25955    25956       +1     
- Misses        1231     1233       +2     

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