feat: be asynchronous

[PaperStrike]

Will be a breaking change but seem to be the only way to support ESLint 8. 😬

May need some new tests for cli coverage.

cc @ljharb

[codecov]

Codecov Report

Merging #13 (a6a74a5) into eslint-8 (b87c9c9) will decrease coverage by 1.01%.
The diff coverage is 92.59%.

❗ Current head a6a74a5 differs from pull request most recent head fa1bfa4. Consider uploading reports for the commit fa1bfa4 to get more accurate results

@@             Coverage Diff              @@
##           eslint-8      #13      +/-   ##
============================================
- Coverage    100.00%   98.98%   -1.02%     
============================================
  Files             9        9              
  Lines           181      197      +16     
============================================
+ Hits            181      195      +14     
- Misses            0        2       +2     

Impacted Files	Coverage Δ
src/bin/find.js	96.42% <50.00%> (-3.58%)	⬇️
src/bin/diff.js	95.45% <66.66%> (-4.55%)	⬇️
src/lib/rule-finder.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b87c9c9...fa1bfa4. Read the comment docs.

[PaperStrike]

Parent: sarbbottam#337

[PaperStrike]

@ljharb Thank you! This PR changes now looks so clean!

There should be no confliction when merging this and #14, since they change different files.