Add keyword capture for charset and add brotli

[boopathi]

Babili uses the charset for mangling from Identifier and StringLiteral. This PR adds the characters from the keywords (const, let, var, class, typeof, void, delete, etc...) to compute the frequency of the characters in the charset to enable better compression.

This computing of frequency somehow increase thes gzip and brotli size.

• Figure out why the size is increasing
• Improve gzip with characters from keyword

Results:

without this patch: (BEFORE)

output raw	brotli output	gzip output
69.14kB	21.33kB	24.05kB

With this patch (AFTER)

output raw	brotli output	gzip output
69.14kB	21.37kB	24.11kB

[codecov]

Codecov Report

Merging #571 into master will decrease coverage by 1.06%.
The diff coverage is 6.52%.

@@            Coverage Diff             @@
##           master     #571      +/-   ##
==========================================
- Coverage   82.93%   81.87%   -1.07%     
==========================================
  Files          41       42       +1     
  Lines        2743     2780      +37     
  Branches      958      970      +12     
==========================================
+ Hits         2275     2276       +1     
- Misses        282      318      +36     
  Partials      186      186

Impacted Files	Coverage Δ
...plugin-minify-mangle-names/src/keyword-visitors.js	2.77% <2.77%> (ø)
...ages/babel-plugin-minify-mangle-names/src/index.js	82.58% <20%> (+0.09%)	⬆️
packages/babel-plugin-minify-simplify/src/index.js	80.61% <0%> (-0.13%)	⬇️

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