build: migrate to eslint flat config as they call it

Wouldn't have managed to do it without the examples found in
sveltejs/eslint-plugin-svelte#732

Makefile

@@ -81,7 +81,7 @@ check-svelte: node_modules
 
 .PHONY: check-eslint
 check-eslint: node_modules
-	$(BIN)/eslint src
+	$(BIN)/eslint src eslint.config.mjs
 
 .PHONY: check-translations
 check-translations:

eslint.config.mjs

@@ -0,0 +1,43 @@
+import eslintPluginSvelte from "eslint-plugin-svelte";
+import globals from "globals";
+import js from "@eslint/js";
+import svelteParser from "svelte-eslint-parser";
+import tsEslint from "typescript-eslint";
+import tsParser from "@typescript-eslint/parser";
+
+export default [
+	js.configs.recommended,
+	...tsEslint.configs.strict,
+	...eslintPluginSvelte.configs["flat/recommended"],
+	{
+		languageOptions: {
+			ecmaVersion: 2020,
+			sourceType: "module",
+		},
+		rules: {
+			"linebreak-style": ["error", "unix"],
+			"semi": ["error", "always"],
+			"comma-dangle": ["error", {
+				arrays: "always-multiline",
+				objects: "always-multiline",
+				imports: "always-multiline",
+				functions: "only-multiline",
+			}],
+			"indent": ["error", "tab"],
+			"quotes": ["warn", "double"],
+			"@typescript-eslint/no-non-null-assertion": "off",
+			"@typescript-eslint/ban-ts-comment": "off",
+		},
+	},
+	{
+		files: ["**/*.svelte"],
+		languageOptions: {
+			parser: svelteParser,
+			globals: globals.browser,
+			parserOptions: {
+				parser: tsParser,
+			},
+		},
+	},
+];
+