Properly handle optional arrow params without type annotations

[alangpierce]

Fixes #387

I reworked typedParseConditional significantly from the Babel implementation
and didn't realize that I needed to preserve bare ? tokens to denote the
parameter optional. Fortunately, the optional case will always be ?, or ?)
(an initializer isn't allowed when the param is optional), and neither of those
cases is valid as the start of a ternary, so we can distinguish optional param
from ternary with only one token of lookahead.

[codecov-io]

Codecov Report

Merging #389 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
+ Coverage    79.8%   79.86%   +0.06%     
==========================================
  Files          45       45              
  Lines        4937     5161     +224     
  Branches     1147     1148       +1     
==========================================
+ Hits         3940     4122     +182     
- Misses        699      739      +40     
- Partials      298      300       +2

Impacted Files	Coverage Δ
src/parser/plugins/types.ts	95% <100%> (+5%)	⬆️
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	80.28% <0%> (-0.77%)	⬇️
src/util/isIdentifier.ts	100% <0%> (ø)	⬆️
src/parser/util/whitespace.ts	100% <0%> (ø)	⬆️
... and 14 more

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[kiliancs]

That was fast! Thank you!