Add support for import assertions

[alangpierce]

Fixes #744
Fixes #566

In ESM mode, we parse import assertions and usually just pass them through to the output code, though TS import elision needs to know how to remove them as well.

In CJS mode, we always remove import assertions since require can handle JSON files.

[codecov]

Codecov Report

Merging #746 (295cee3) into main (0ae8d9b) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #746      +/-   ##
==========================================
+ Coverage   87.43%   87.51%   +0.08%     
==========================================
  Files          54       55       +1     
  Lines        5856     5887      +31     
  Branches     1389     1394       +5     
==========================================
+ Hits         5120     5152      +32     
  Misses        466      466              
+ Partials      270      269       -1     

Impacted Files	Coverage Δ
src/parser/tokenizer/readWordTree.ts	100.00% <ø> (ø)
src/TokenProcessor.ts	95.16% <100.00%> (+0.37%)	⬆️
src/parser/tokenizer/keywords.ts	100.00% <100.00%> (ø)
src/parser/traverser/statement.ts	83.58% <100.00%> (+0.13%)	⬆️
src/transformers/CJSImportTransformer.ts	88.55% <100.00%> (+0.14%)	⬆️
src/transformers/ESMImportTransformer.ts	94.87% <100.00%> (+0.75%)	⬆️
src/util/removeMaybeImportAssertion.ts	100.00% <100.00%> (ø)

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[github-actions]

Benchmark results

Before this PR: 392.1 thousand lines per second
After this PR: 394.8 thousand lines per second

Measured change: 0.7% faster (0.36% slower to 1.18% faster)
Summary: Likely no significant difference