Copy linter configuration from tc39/proposal-temporal

This wasn't in the polyfill/ directory so it didn't get copied in the
fork. Re-import this linter configuration (with some adjustments to the
paths)

Author: ptomato

.eslintrc.yml

@@ -0,0 +1,84 @@
+env:
+  browser: true
+  es6: true
+  node: true
+plugins:
+  - prettier
+  - '@typescript-eslint'
+extends:
+  - 'eslint:recommended'
+  - 'plugin:prettier/recommended'
+  - 'plugin:@typescript-eslint/recommended'
+parser: '@typescript-eslint/parser' # for index.d.ts
+globals:
+  Atomics: readonly
+  BigInt: readonly
+  SharedArrayBuffer: readonly
+  globalThis: readonly
+parserOptions:
+  ecmaVersion: 2020
+  sourceType: module
+ignorePatterns:
+  - node_modules/
+  - /dist/
+rules:
+  array-element-newline:
+    - error
+    - consistent
+  arrow-parens: error
+  arrow-spacing: error
+  brace-style:
+    - error
+    - 1tbs
+  comma-dangle: error
+  comma-spacing: error
+  curly:
+    - error
+    - multi-line
+  func-call-spacing: error
+  function-call-argument-newline:
+    - error
+    - consistent
+  indent:
+    - error
+    - 2
+    - SwitchCase: 1
+  keyword-spacing: error
+  max-len:
+    - error
+    - code: 120
+      ignoreRegExpLiterals: true
+  no-alert: error
+  no-console: error
+  no-multiple-empty-lines:
+    - error
+    - max: 1
+  no-trailing-spaces: error
+  object-curly-spacing:
+    - error
+    - always
+  object-property-newline:
+    - error
+    - allowAllPropertiesOnSameLine: true
+  quote-props:
+    - error
+    - as-needed
+  quotes:
+    - error
+    - single
+    - avoidEscape: true
+  semi: error
+  space-infix-ops: error
+  '@typescript-eslint/explicit-module-boundary-types': off
+  '@typescript-eslint/no-empty-function': off
+  '@typescript-eslint/no-var-requires': off
+  '@typescript-eslint/ban-ts-comment': off
+overrides:
+  - files:
+      - lib/duration.mjs
+      - lib/init.js
+      - test/all.mjs
+      - test/exhaust.mjs
+      - test/validStrings.mjs
+    rules:
+      no-console: off

.gitignore

@@ -1,3 +1,4 @@
 package-lock.json
 node_modules/
 dist/
+.eslintcache

package.json

@@ -10,7 +10,10 @@
     "test": "node --no-warnings --experimental-modules --icu-data-dir node_modules/full-icu --loader ./test/resolve.source.mjs ./test/all.mjs",
     "build": "rollup -c rollup.config.js",
     "prepublishOnly": "npm run build",
-    "playground": "node --experimental-modules --no-warnings --icu-data-dir node_modules/full-icu -r ./lib/init.js"
+    "playground": "node --experimental-modules --no-warnings --icu-data-dir node_modules/full-icu -r ./lib/init.js",
+    "lint": "eslint . --ext js,mjs,.d.ts --max-warnings 0 --cache \"$@\"",
+    "postlint": "npm run tscheck",
+    "tscheck": "tsc index.d.ts --noEmit --strict --lib ESNext"
   },
   "keywords": [
     "Calendar",
@@ -60,12 +63,28 @@
     "@rollup/plugin-commonjs": "^15.1.0",
     "@rollup/plugin-node-resolve": "^9.0.0",
     "@rollup/plugin-replace": "^2.3.3",
+    "@typescript-eslint/eslint-plugin": "^4.3.0",
+    "@typescript-eslint/parser": "^4.3.0",
+    "eslint": "^7.10.0",
+    "eslint-config-prettier": "^6.12.0",
+    "eslint-plugin-prettier": "^3.1.4",
     "full-icu": "^1.3.0",
+    "prettier": "^2.1.2",
     "rollup": "^2.28.2",
     "rollup-plugin-terser": "^7.0.2",
-    "timezones.json": "^1.5.2"
+    "timezones.json": "^1.5.2",
+    "typescript": "^4.0.3"
   },
   "engines": {
     "node": ">=12"
+  },
+  "prettier": {
+    "printWidth": 120,
+    "trailingComma": "none",
+    "tabWidth": 2,
+    "semi": true,
+    "singleQuote": true,
+    "bracketSpacing": true,
+    "arrowParens": "always"
   }
 }