Build: Migrate to the ESLint flat config

Closes gh-610

Author: mgol

.eslintignore

@@ -0,0 +1,85 @@
+import jqueryConfig from "eslint-config-jquery";
+import globals from "globals";
+
+export default [
+	{
+		linterOptions: {
+			reportUnusedDisableDirectives: "error"
+		}
+	},
+
+	{
+		ignores: [
+			"app/dist/**",
+			"external/**",
+			"jquery-ui/**",
+			"template/**",
+			"test/fixtures/**",
+			"tmp/**",
+			"*.min.js"
+		]
+	},
+
+	{
+		languageOptions: {
+			sourceType: "commonjs",
+			ecmaVersion: 2022,
+			parserOptions: {
+				globalReturn: true
+			},
+			globals: {
+				...globals.es2021,
+				...globals.node
+			}
+		},
+		rules: {
+			...jqueryConfig.rules,
+			strict: [ "error", "global" ],
+
+			// Too many violations
+			"camelcase": "off",
+			"max-len": "off",
+			"no-unused-vars": "off"
+		}
+	},
+
+	{
+		files: [ "*.mjs" ],
+		languageOptions: {
+			sourceType: "module"
+		}
+	},
+
+	{
+		files: [ "test/*.js" ],
+		languageOptions: {
+			globals: {
+				...globals.es2021,
+				...globals.node,
+				...globals.mocha
+			}
+		}
+	},
+
+	{
+		files: [ "app/src/**/*.js" ],
+		languageOptions: {
+			// No need to keep IE support, so we could bump it to ES2022 as well,
+			// but we need to switch the minifier to something other than UglifJS
+			// which is ES5-only.
+			ecmaVersion: 5,
+			sourceType: "script",
+			parserOptions: {
+				globalReturn: false
+			},
+			globals: {
+				...globals.es2021,
+				...globals.browser,
+				jQuery: false
+			}
+		},
+		rules: {
+			"strict": [ "error", "function" ]
+		}
+	}
+];