Properly elide TS imports even when referenced by shadowing variables

[alangpierce]

Fixes #298

TypeScript is required to remove imports where all bindings are never referenced
in a value position, but previously it could have false positives when there are
shadowed variables with the same name. Now, we do the intelligent scope-based
shadowed global detection like we do with import replacement and properly elide
when necessary.

[codecov-io]

Codecov Report

Merging #342 into master will decrease coverage by 0.06%.
The diff coverage is 82.22%.

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
- Coverage   77.47%   77.41%   -0.07%     
==========================================
  Files          41       42       +1     
  Lines        5305     5529     +224     
  Branches     1289     1302      +13     
==========================================
+ Hits         4110     4280     +170     
- Misses        827      878      +51     
- Partials      368      371       +3

Impacted Files	Coverage Δ
src/util/getNonTypeIdentifiers.ts	100% <ø> (+25%)	⬆️
src/index.ts	89.36% <100%> (+4.51%)	⬆️
src/util/getTSImportedNames.ts	80.48% <80.48%> (ø)
src/util/formatTokens.ts	70.58% <0%> (-10.06%)	⬇️
src/parser/plugins/flow.ts	58.46% <0%> (-2.35%)	⬇️
src/parser/plugins/jsx/index.ts	89.11% <0%> (-2.07%)	⬇️
src/parser/tokenizer/readWord.ts	70.35% <0%> (-1.47%)	⬇️
src/util/getClassInfo.ts	85.12% <0%> (-1.21%)	⬇️
src/parser/traverser/statement.ts	80.14% <0%> (-0.84%)	⬇️
src/parser/traverser/lval.ts	95.94% <0%> (-0.78%)	⬇️
... and 16 more

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