Add support for basic TS assertion signature syntax

[alangpierce]

Fixes #503

The previous code didn't handle the basic case asserts x, which I just
overlooked when backporting Babel parser changes. The new code has a special
case for that and conditionally tells the caller whether to parse a type
afterward. I also made the contract a little simpler; it either fully parses the
return type or it doesn't.

[codecov-io]

Codecov Report

Merging #504 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #504     +/-   ##
=========================================
- Coverage   82.68%   82.59%   -0.1%     
=========================================
  Files          55       55             
  Lines        5608     5383    -225     
  Branches     1261     1263      +2     
=========================================
- Hits         4637     4446    -191     
+ Misses        681      650     -31     
+ Partials      290      287      -3

Impacted Files	Coverage Δ
src/parser/plugins/typescript.ts	80.82% <100%> (+0.23%)	⬆️
src/Options-gen-types.ts	75% <0%> (-25%)	⬇️
src/util/getDeclarationInfo.ts	80% <0%> (-20%)	⬇️
src/util/elideImportEquals.ts	81.81% <0%> (-18.19%)	⬇️
src/util/isAsyncOperation.ts	82.35% <0%> (-17.65%)	⬇️
src/util/shouldElideDefaultExport.ts	68.75% <0%> (-9.03%)	⬇️
src/parser/index.ts	77.77% <0%> (-7.94%)	⬇️
src/computeSourceMap.ts	83.33% <0%> (-5.56%)	⬇️
src/util/getNonTypeIdentifiers.ts	94.44% <0%> (-5.56%)	⬇️
src/parser/plugins/types.ts	91.66% <0%> (-3.34%)	⬇️
... and 20 more

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