First pass at implementation of optional chaining

[alangpierce]

Progress toward #461

Tech plan: https://github.com/alangpierce/sucrase/wiki/Sucrase-Optional-Chaining-and-Nullish-Coalescing-Technical-Plan

The parser now associates each access operation with the start token for the
chain, and marks each chain containing an optional chain operation so that we
can wrap it in a function call.

This does not yet handle optional deletion, and there are a number of follow-up
tests that would be good to write, but the feature seems to be working with this
implementation.

[codecov-io]

Codecov Report

Merging #490 into master will increase coverage by 0.14%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   82.28%   82.42%   +0.14%     
==========================================
  Files          53       54       +1     
  Lines        5464     5508      +44     
  Branches     1214     1225      +11     
==========================================
+ Hits         4496     4540      +44     
  Misses        681      681              
  Partials      287      287

Impacted Files	Coverage Δ
src/HelperManager.ts	100% <ø> (ø)	⬆️
src/transformers/RootTransformer.ts	92.38% <100%> (-0.04%)	⬇️
src/parser/plugins/typescript.ts	80.36% <100%> (ø)	⬆️
src/parser/tokenizer/index.ts	77.31% <100%> (+0.15%)	⬆️
src/TokenProcessor.ts	91.66% <100%> (+0.45%)	⬆️
src/parser/plugins/flow.ts	63.72% <66.66%> (ø)	⬆️
...transformers/OptionalChainingNullishTransformer.ts	85.18% <85.18%> (ø)
src/parser/traverser/expression.ts	86.96% <86.95%> (+1.15%)	⬆️

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