add test coverage and check html files on watch mode

.gitignore

@@ -1,3 +1,4 @@
 .vscode
 node_modules
 lib
+coverage

jest.config.js

@@ -1,4 +1,8 @@
+const { defaults } = require("jest-config");
+
 /** @type {import('@jest/types').Config.InitialOptions} */
 module.exports = () => ({
 	preset: "ts-jest",
+	moduleFileExtensions: [...defaults.moduleFileExtensions, "html"],
+	collectCoverage: true,
 });