Type-check tokenizer/types.js

[ghost]

Q	A
Bug fix?	no
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	no
Tests added/pass?	Tests pass
Fixed tickets
License	MIT

Enables type-checking in the file and adds type annotations.

Also changed the implementation of unexpected to something that would type-check. It should be equivalent given the allowed inputs; we don't need to test for non-nullity or for the presence of .label assuming callers respect the type.

[codecov]

Codecov Report

Merging #493 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #493   +/-   ##
======================================
  Coverage    98.2%   98.2%           
======================================
  Files          21      21           
  Lines        3517    3517           
  Branches      974     974           
======================================
  Hits         3454    3454           
  Misses         24      24           
  Partials       39      39

Flag	Coverage Δ
#babel	80.89% <100%> (ø)	⬆️
#babylon	97.04% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/tokenizer/context.js	100% <ø> (ø)	⬆️
src/tokenizer/types.js	100% <100%> (ø)	⬆️
src/parser/util.js	90% <100%> (ø)	⬆️

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