Add eslint support.

Run with `yarn run lint` or `bin/webpack`.

.eslintrc.yml

@@ -0,0 +1,88 @@
+env:
+  browser: true
+  es6: true
+extends: 'eslint:recommended'
+parserOptions:
+  sourceType: module
+globals:
+  $: false
+  require: false
+rules:
+  # https://eslint.org/docs/rules/
+  array-callback-return: error
+  block-scoped-var: error
+  consistent-return: error
+  default-case: error
+  dot-notation: error
+  eqeqeq: error
+  init-declarations: error
+  no-caller: error
+  no-empty-function: error
+  no-eval: error
+  no-extend-native: error
+  no-implicit-coercion: error
+  no-lone-blocks: error
+  no-lonely-if: error
+  no-mixed-operators: error
+  no-new: error
+  no-new-wrappers: error
+  no-return-assign: error
+  no-self-compare: error
+  no-sequences: error
+  no-shadow: error
+  no-shadow-restricted-names: error
+  no-unused-expressions: error
+  no-unused-vars:
+    - error
+    - argsIgnorePattern: "^_"
+      args: none
+  no-use-before-define: error
+  no-useless-call: error
+  no-useless-concat: error
+  no-useless-return: error
+
+  # formatting issues
+  array-bracket-spacing: warn
+  brace-style: [warn, 1tbs, allowSingleLine: true]
+  comma-spacing: warn
+  curly: warn
+  dot-location: [warn, property]
+  eol-last: warn
+  func-call-spacing: warn
+  indent: [warn, 2]
+  linebreak-style: [warn, unix]
+  key-spacing: warn
+  keyword-spacing: warn
+  no-multiple-empty-lines: warn
+  no-tabs: warn
+  no-trailing-spaces: warn
+  no-whitespace-before-property: warn
+  space-before-blocks: warn
+  space-in-parens: warn
+  space-infix-ops: warn
+  space-unary-ops: warn
+  spaced-comment: warn
+
+# These rules are potentially useful, but are currently too noisy to enable.
+
+#  block-spacing: warn
+#  no-alert: warn
+#  no-extra-parens: warn
+#  no-magic-numbers:
+#    - warn
+#    - ignore: [-1, 0, 1]
+#      ignoreArrayIndexes: true
+#      enforceConst: true
+#  no-multi-spaces: warn
+#  no-negated-condition: warn
+#  no-param-reassign: warn
+#  no-var: warn
+#  object-curly-spacing: warn
+#  prefer-arrow-callback: warn
+#  prefer-const: warn
+#  prefer-template: warn
+#  quote-props: [warn, consistent-as-needed]
+#  radix: warn
+#  semi: warn
+#  space-before-function-paren: warn
+#  vars-on-top: warn