Add keepUnusedImports option

[alangpierce]

Supersedes #615
Fixes #791

This PR builds off of #615 with a number of additional tests and cases. High-level, it introduces a new option keepUnusedImports that disables all automatic import and export elision done by TypeScript, somewhat analogous to the verbatimModuleSyntax TS option. For the most part, this is just a matter of flagging off the name tracking and elision code
for both imports and exports in the various places that they happen.

When this mode is enabled, some of the preprocessing steps can be skipped, and scope handling can be fully skipped with
the imports transform also disabled. Performance numbers measured by yarn benchmark jest-dev:

• With imports and typescript transforms: 855k -> 910k lines per second (6% faster)
• With only typescript transform: 935k -> 1110k lines per second (19% faster)

Some additional details:

• While this feature is intended to be used with TypeScript, I decided to also implement the behavior for Flow as well. Flow's implementation in Babel will remove an import statement if all individual names are removed, but flow-remove-types won't do this, so Sucrase keepUnusedImports matches the behavior of flow-remove-types.
• Babel doesn't currently implement this option, so for the playground I mapped it to onlyRemoveTypeImports, which
  seems to have the closest behavior.

[codecov]

Codecov Report

Merging #811 (0b6e699) into main (a924d84) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #811      +/-   ##
==========================================
+ Coverage   88.68%   88.70%   +0.01%     
==========================================
  Files          56       56              
  Lines        6073     6082       +9     
  Branches     1444     1446       +2     
==========================================
+ Hits         5386     5395       +9     
  Misses        421      421              
  Partials      266      266              

Impacted Files	Coverage Δ
src/Options-gen-types.ts	100.00% <ø> (ø)
src/Options.ts	100.00% <ø> (ø)
src/transformers/RootTransformer.ts	93.39% <ø> (ø)
src/CJSImportProcessor.ts	91.95% <100.00%> (+0.04%)	⬆️
src/index.ts	92.15% <100.00%> (ø)
src/transformers/CJSImportTransformer.ts	90.33% <100.00%> (+0.06%)	⬆️
src/transformers/ESMImportTransformer.ts	96.77% <100.00%> (+0.08%)	⬆️
src/util/shouldElideDefaultExport.ts	88.88% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[github-actions]

Benchmark results

Before this PR: 329.8 thousand lines per second
After this PR: 326 thousand lines per second

Measured change: 1.17% slower (2.1% slower to 0.26% slower)
Summary: Probably slower