perf(ts-estree): don't create Program in parse()

[JamesHenry]

Improves the performance for the parse() use case - e.g. Prettier's current usage.

From the existing benchmark it seems to be ~50% speed increase gained by not bothering with a full ts.Program and just creating a ts.SourceFile directly.

@typescript-eslint/benchmark: original-parse x 7,332 ops/sec ±6.34% (74 runs sampled)
@typescript-eslint/benchmark: updated-parse x 11,401 ops/sec ±1.54% (87 runs sampled)

I also streamlined some things and added more inline documentation

[codecov]

Codecov Report

Merging #148 into master will increase coverage by 0.38%.
The diff coverage is 96.61%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   95.31%   95.69%   +0.38%     
==========================================
  Files          49       49              
  Lines        2477     2487      +10     
  Branches      370      373       +3     
==========================================
+ Hits         2361     2380      +19     
+ Misses         69       63       -6     
+ Partials       47       44       -3

Impacted Files	Coverage Δ
packages/typescript-estree/src/parser.ts	91.3% <96.61%> (+9.39%)	⬆️

[JamesHenry]

@uniqueiniquity FYI after I made these gains with the parse() use-case I tried to keep going and improve the parseAndGenerateServices() that we use in linting, but I'm afraid I didn't make any significant progress yet

[uniqueiniquity]

Makes sense to me. For syntactic stuff I don’t see any reason to have the ts.Program.