Add null as reserved word in enum emit, other follow-ups

[alangpierce]

Follow-up to #656:

• Add null as a new case that should not have a variable declared in enum
  emit.
• Rearrange the declaration to group these special non-identifier literals.
• Add a test, also confirming that undefined is not reserved here.
• Add a jsdoc comment for isIdentifier.

[codecov]

Codecov Report

Merging #657 (779cf34) into main (6aba006) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #657      +/-   ##
==========================================
- Coverage   82.65%   82.58%   -0.08%     
==========================================
  Files          56       56              
  Lines        5928     5690     -238     
  Branches     1344     1344              
==========================================
- Hits         4900     4699     -201     
+ Misses        745      708      -37     
  Partials      283      283              

Impacted Files	Coverage Δ
src/util/isIdentifier.ts	100.00% <ø> (ø)
src/Options-gen-types.ts	75.00% <0.00%> (-25.00%)	⬇️
src/parser/index.ts	77.77% <0.00%> (-7.94%)	⬇️
src/computeSourceMap.ts	83.33% <0.00%> (-5.56%)	⬇️
src/util/getNonTypeIdentifiers.ts	94.44% <0.00%> (-5.56%)	⬇️
src/parser/plugins/types.ts	91.66% <0.00%> (-3.34%)	⬇️
src/parser/tokenizer/readWord.ts	82.35% <0.00%> (-3.02%)	⬇️
src/parser/traverser/util.ts	83.33% <0.00%> (-2.95%)	⬇️
src/parser/traverser/base.ts	95.45% <0.00%> (-1.69%)	⬇️
src/parser/traverser/statement.ts	82.16% <0.00%> (-0.44%)	⬇️
... and 20 more

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[github-actions]

Benchmark results

Before this PR: 266 thousand lines per second
After this PR: 270.9 thousand lines per second

Measured change: 1.85% faster (0.66% faster to 9.14% faster)
Summary: Probably faster

[alangpierce]

Published as 3.20.3.