Lit-html: Replace TSLint with ESLint

[abdonrd]

Ad TSLint is being deprecated in favor of ESLint, I want to propose to replace it.
Also, good moment to add pre-commit hook for linting and format.

References

• Palantir Blog: TSLint in 2019
• TypeScript Roadmap: January - June 2019
• ESLint Blog: The future of TypeScript on ESLint

Proposal

Use @typescript-eslint, husky and lint-staged.

package.json

  "devDependencies": {
    "@typescript-eslint/eslint-plugin": "^2.0.0",
    "@typescript-eslint/parser": "^2.0.0",
    "eslint": "^6.2.0",
    "husky": "^3.0.0",
    "lint-staged": "^9.2.0"
  },
  "husky": {
    "hooks": {
      "pre-commit": "lint-staged"
    }
  },
  "lint-staged": {
    "**/*.{js,ts}": [
      "eslint --fix",
      "git add"
    ]
  }

.eslintrc.json

{
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/eslint-recommended",
    "plugin:@typescript-eslint/recommended"
  ],
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "ecmaVersion": 2018,
    "sourceType": "module"
  },
  "plugins": [
    "@typescript-eslint"
  ]
}

.eslintignore

/lib/
/directives/
/polyfills/
/test/**/*.{d.ts,d.ts.map,js,js.map}
/tools/lib/
/lit-html.{d.ts,d.ts.map,js,js.map}

[abdonrd]

I have created a draft PR (#969), if you like it I can continue working on it.

[justinfagnani]

We still use tslint internally at Google, and it sounds like it'll be awhile before we can move off it. I'm not sure how much we gain from keeping the same tooling, but being able to move contributions between systems is nice. cc @rictic and @aomarks

[rictic]

If there are desirable lint passes in ESLint that aren't in TSLint then I say go for it. Lint errors won't block syncing code and they don't break the build so it's ok if they get a little out of sync.

[abdonrd]

Also @aomarks already merged ESLint in the Material Web Components project.