Introduce ESLint to svelte sources (#2958)

* introduce eslint

* add lint npm script

* first eslint run

* use tsc stronger checks for unused vars

* fix strict typescript unused checks

* reintroduce unicode oddities

* revert ordering changes

* revert whitespace changes

* set indent lint rule as error and ignore outliers

* revert some more unicode

* reintroduce scissors unicode again

.eslintignore

@@ -1,7 +1,3 @@
-src/shared
-shared.js
-store.js
-test/test.js
-test/setup.js
 **/_actual.js
-**/expected.js
+**/expected.js
+test/*/samples/*/output.js

.eslintrc.json

@@ -1,43 +1,76 @@
 {
-	"root": true,
-	"rules": {
-		"indent": [2, "tab", { "SwitchCase": 1 }],
-		"semi": [2, "always"],
-		"keyword-spacing": [2, { "before": true, "after": true }],
-		"space-before-blocks": [2, "always"],
-		"no-mixed-spaces-and-tabs": [2, "smart-tabs"],
-		"no-cond-assign": 0,
-		"no-unused-vars": 2,
-		"object-shorthand": [2, "always"],
-		"no-const-assign": 2,
-		"no-class-assign": 2,
-		"no-this-before-super": 2,
-		"no-var": 2,
-		"no-unreachable": 2,
-		"valid-typeof": 2,
-		"quote-props": [2, "as-needed"],
-		"one-var": [2, "never"],
-		"prefer-arrow-callback": 2,
-		"prefer-const": [2, { "destructuring": "all" }],
-		"arrow-spacing": 2,
-		"no-inner-declarations": 0
-	},
-	"env": {
-		"es6": true,
-		"browser": true,
-		"node": true,
-		"mocha": true
-	},
-	"extends": [
-		"eslint:recommended",
-		"plugin:import/errors",
-		"plugin:import/warnings"
-	],
-	"parserOptions": {
-		"ecmaVersion": 9,
-		"sourceType": "module"
-	},
-	"settings": {
-		"import/core-modules": ["svelte"]
-	}
+  "root": true,
+  "rules": {
+    "indent": "off",
+    "no-unused-vars": "off",
+    "semi": [2, "always"],
+    "keyword-spacing": [2, { "before": true, "after": true }],
+    "space-before-blocks": [2, "always"],
+    "no-mixed-spaces-and-tabs": [2, "smart-tabs"],
+    "no-cond-assign": 0,
+    "object-shorthand": [2, "always"],
+    "no-const-assign": 2,
+    "no-class-assign": 2,
+    "no-this-before-super": 2,
+    "no-var": 2,
+    "no-unreachable": 2,
+    "valid-typeof": 2,
+    "quote-props": [2, "as-needed"],
+    "one-var": [2, "never"],
+    "prefer-arrow-callback": 2,
+    "prefer-const": [2, { "destructuring": "all" }],
+    "arrow-spacing": 2,
+    "no-inner-declarations": 0,
+    "@typescript-eslint/indent": ["error", "tab", {
+      "SwitchCase": 1,
+      "ignoredNodes": ["TemplateLiteral"]
+    }],
+    "@typescript-eslint/camelcase": "off",
+    "@typescript-eslint/no-use-before-define": "off",
+    "@typescript-eslint/array-type": ["error", "array-simple"],
+    "@typescript-eslint/explicit-function-return-type": "off",
+    "@typescript-eslint/no-explicit-any": "off",
+    "@typescript-eslint/explicit-member-accessibility": "off",
+    "@typescript-eslint/no-unused-vars": ["error", {
+      "argsIgnorePattern": "^_"
+    }],
+    "@typescript-eslint/no-object-literal-type-assertion": ["error", {
+      "allowAsParameter": true
+    }],
+    "@typescript-eslint/no-unused-vars": "off"
+  },
+  "env": {
+    "es6": true,
+    "browser": true,
+    "node": true,
+    "mocha": true
+  },
+  "extends": [
+    "eslint:recommended",
+    "plugin:import/errors",
+    "plugin:import/warnings",
+    "plugin:import/typescript",
+    "plugin:@typescript-eslint/recommended"
+  ],
+  "parserOptions": {
+    "ecmaVersion": 9,
+    "sourceType": "module"
+  },
+  "settings": {
+    "import/core-modules": [
+      "svelte",
+      "svelte/internal",
+      "svelte/store",
+      "svelte/easing",
+      "estree"
+    ]
+  },
+  "overrides": [
+    {
+      "files": ["*.js"],
+      "rules": {
+        "@typescript-eslint/no-var-requires": "off"
+      }
+    }
+  ]
 }