[HTML5] Enforce JavaScript style with eslint.

Applies to javascript files inside the platform library folder, the
exposed Engine code, and any javascript files in modules.

Files ending with ".externs.js" will be ignored, you can create a
".eslintignore" file to specify extra files to be ignored.

platform/javascript/.eslintrc.js

@@ -0,0 +1,43 @@
+module.exports = {
+	"env": {
+		"browser": true,
+		"es2021": true,
+	},
+	"extends": [
+		"airbnb-base",
+	],
+	"parserOptions": {
+		"ecmaVersion": 12,
+	},
+	"ignorePatterns": "*.externs.js",
+	"rules": {
+		"func-names": "off",
+		// Use tabs for consistency with the C++ codebase.
+		"indent": ["error", "tab"],
+		"max-len": "off",
+		"no-else-return": ["error", {allowElseIf: true}],
+		"curly": ["error", "all"],
+		"brace-style": ["error", "1tbs", { "allowSingleLine": false }],
+		"no-bitwise": "off",
+		"no-continue": "off",
+		"no-self-assign": "off",
+		"no-tabs": "off",
+		"no-param-reassign": ["error", { "props": false }],
+		"no-plusplus": "off",
+		"no-unused-vars": ["error", { "args": "none" }],
+		"prefer-destructuring": "off",
+		"prefer-rest-params": "off",
+		"prefer-spread": "off",
+		"camelcase": "off",
+		"no-underscore-dangle": "off",
+		"max-classes-per-file": "off",
+		"prefer-arrow-callback": "off",
+		// Messes up with copyright headers in source files.
+		"spaced-comment": "off",
+		// Completely breaks emscripten libraries.
+		"object-shorthand": "off",
+		// Closure compiler (exported properties)
+		"quote-props": ["error", "consistent"],
+		"dot-notation": "off",
+	}
+};