Migrate to ESLint

Created by: felixfbecker

Closes #2461

Very excited about this one as ESLint has support for a bunch of new rules that we can now check for that often came up in code reviews, e.g. referencing this.state inside setState() and many more.

I recreated our TSLint config in https://github.com/sourcegraph/eslint-config and made https://github.com/sourcegraph/tslint-config/tree/eslint contain only the few TSLint rules ESLint doesn't have.

Migratingwas more work than expected but it's a huge improvement to our codebase, e.g. finally refactored all usages of async-unsafe componentWillReceiveProps() to componentDidUpdate(), removing unused state properties, ...

Also like the better configurability of ESLint - configuring rules for glob patterns or overriding severity in comments.

I also noticed we were not typechecking our code in CI! So this fixes it.

cc @JoshuaKGoldberg

Created by: codecov[bot]

Codecov Report

Merging #5072 into master will increase coverage by 0.04%. The diff coverage is 54.14%.

@@            Coverage Diff             @@
##           master    #5072      +/-   ##
==========================================
+ Coverage   46.27%   46.32%   +0.04%     
==========================================
  Files         732      731       -1     
  Lines       44909    44872      -37     
  Branches     2613     2602      -11     
==========================================
+ Hits        20782    20787       +5     
+ Misses      22115    22072      -43     
- Partials     2012     2013       +1

Impacted Files	Coverage Δ
shared/src/commandPalette/CommandList.tsx	21.18% <ø> (ø)
browser/src/libs/options/OptionsHeader.tsx	100% <ø> (ø)
shared/src/api/client/services/decoration.ts	63.63% <ø> (ø)
browser/src/libs/options/ServerURLForm.tsx	89.74% <ø> (ø)
shared/src/api/extension/api/types.ts	90% <ø> (ø)
shared/src/util/errors.ts	63.63% <ø> (ø)
shared/src/components/RepoLink.tsx	100% <ø> (ø)
...rowser/src/libs/code_intelligence/test_helpers.tsx	80% <ø> (ø)
shared/src/panel/views/contributions.ts	100% <ø> (ø)
...nsions/extension/RegistryExtensionOverviewPage.tsx	74.07% <ø> (ø)
... and 108 more

Created by: felixfbecker

It would be great to get a timely review from someone frontend (currently @chrismwendt or @sqs) since this very prone to merge conflicts. At the same time I could have messed up a refactor somewhere. :)

Created by: JoshuaKGoldberg

Heh, the GitHub editor keeps freezing up my Chrome tab because of how many files there are (have you considered splitting out source file changes where possible?).

But, this brings up a super cool idea: would it have been convenient to have a tool that converts tslint:disable comments to eslint-disable equivalents?

Created by: felixfbecker

But, this brings up a super cool idea: would it have been convenient to have a tool that converts tslint:disable comments to eslint-disable equivalents?

Yes, 100%! :)

Created by: felixfbecker

Do we no longer disallow submodule imports?

We didn't allow submodule imports with TSLint because a package's index.ts should export the public API. Importing from deep files is importing internals - if we do that then we cannot confidently evolve our packages anymore. This has been a problem with internal packages in the past.

Is ?(x) valid glob syntax? See lsif/server/package.json

Yep, see https://github.com/isaacs/node-glob#glob-primer. What do you mean with "See lsif/server/package.json"?