Treat ! tokens as type tokens in TS variable declarations

[alangpierce]

Fixes #406

[codecov-io]

Codecov Report

Merging #410 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   80.03%   80.06%   +0.02%     
==========================================
  Files          46       46              
  Lines        4958     5186     +228     
  Branches     1155     1155              
==========================================
+ Hits         3968     4152     +184     
- Misses        695      737      +42     
- Partials      295      297       +2

Impacted Files	Coverage Δ
src/parser/plugins/typescript.ts	75.03% <100%> (+0.07%)	⬆️
src/util/formatTokens.ts	70.58% <0%> (-10.06%)	⬇️
src/parser/util/identifier.ts	90.9% <0%> (-4.1%)	⬇️
src/parser/plugins/jsx/index.ts	87.33% <0%> (-1.88%)	⬇️
src/parser/plugins/flow.ts	62.21% <0%> (-1.59%)	⬇️
src/util/getClassInfo.ts	85.12% <0%> (-1.21%)	⬇️
src/identifyShadowedGlobals.ts	87.8% <0%> (-1.09%)	⬇️
src/parser/traverser/statement.ts	80.46% <0%> (-0.78%)	⬇️
src/util/isIdentifier.ts	100% <0%> (ø)	⬆️
src/parser/util/whitespace.ts	100% <0%> (ø)	⬆️
... and 15 more

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