Add prettier + eslint

[fathyb]

This PR adds prettier-eslint, which is just prettier with its output piped to eslint --fix. js files are formatted using eslint rules, while md and json files only using prettier.

husky is added to setup a Git precommit hook, and lint-staged to only check staged files. This will make sure every commit is properly formatted and passes the lint test. This can be bypassed by using the Git commit --no-verify option.

A format script is added to run prettier on all files.

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https://segment.atlassian.net/browse/LIB-413
cc @f2prateek

[fathyb]

Had to upgrade to 8 for lint-staged, see CI Job#14

[f2prateek]

Would this have any effect on the final functionality?

[fathyb]

The final artifact is built by the analytics.js repo so it should only affect tests

[f2prateek]

Do you think we still need to keep the old config around?

[fathyb]

I think we should, browser/legacy ensures we are only using ES3 compatible code which we can't really guarantee otherwise given we don't use any transpilers

[f2prateek]

Gotcha, that makes sense - we definitely need that 🙏

[f2prateek]

If we're not using ESLint anymore in the makefile, we could remove it from line 5 as well.

[fathyb]

Good catch, fixed

[fathyb]

Today's free tip: with newer versions of yarn you can directly invoke bin programs.
Example: yarn codecov instead of ./node_modules/.bin/codecov

[f2prateek]

Is this needed? What does it do? I see our internal app codebase using single quotes but not have this prettier rule.

[f2prateek]

Ah nvm, they pass it as an option in the binary instead: /prettier --write --single-quote --no-semi 'src/**/*.js'. I like putting it the config better 👍

[fathyb]

If we remove it prettier will use double quotes when formatting. It could also be in .eslintrc and package.json IIRC. I've put it in this file so editor plugins can use this setting.

[f2prateek]

couple of questions but looks great overall.