(ci): add a lint job so PRs will require passing lint

[agilgur5]

• a few recent PRs got in without passing lint, this breaks all future
  PRs/branches/commits because pre-commit will fail when it runs lint

Turns out lint was never running in CI! Which explains why #370 and #366 passed (and my confusion in #373 )
This should make formatting fixes like #372 and #377 unnecessary in the future if only PRs that pass lint are merged.

I limited this to run on Node v10 and ubuntu-latest -- not sure if a matrix is necessary for self-linting. The existing test suite already has tests for tsdx lint and that runs over the matrix ofc.

[agilgur5]

Ok, this is a bit weird that it passes lint prior to #377 being merged...

The error I was getting was:

yarn run v1.19.1
$ yarn build && yarn lint:post-build
$ tsc -p tsconfig.json
$ node dist/index.js lint src test --ignore-pattern 'test/tests/lint'
=============

WARNING: You are currently running a version of TypeScript which is not officially supported by typescript-estree.

You may find that it works just fine, or you may not.

SUPPORTED TYPESCRIPT VERSIONS: >=3.2.1 <3.7.0

YOUR TYPESCRIPT VERSION: 3.7.3

Please only submit bug reports when using the officially supported version.

=============

~/Desktop/GitHub/tsdx/test/fixtures/build-default/src/syntax/nullish-coalescing.ts
  5:24  error  Parsing error: Expression expected  prettier/prettier

~/Desktop/GitHub/tsdx/test/fixtures/build-default/src/syntax/optional-chaining.ts
  3:52  error  Parsing error: Expression expected  prettier/prettier

✖ 2 problems (2 errors, 0 warnings)

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

But then the prettier errors stopped happening once I added the fix, including on local, despite that not being part of this branch... 🤔

[swyxio]

great calls in every respect.

[agilgur5]

Ah, I see, #370 added a commit later which bumped prettier which bumped its dependency on @typescript-eslint/typescript-estree, so it then started to pass lint when deps were upgraded.

#377 was still needed to get rid of the big warning and to support optional chaining / nullish coalescing in eslint proper (not just in prettier), but not to fix the specific prettier errors I wrote of above.