test: add vitest and svelte testing library (resolves #13)

vitest.config.ts

@@ -0,0 +1,12 @@
+import { defineConfig } from 'vitest/config';
+import { sveltekit } from '@sveltejs/kit/vite';
+import { svelteTesting } from '@testing-library/svelte/vite';
+
+export default defineConfig({
+	plugins: [sveltekit(), svelteTesting()],
+	test: {
+		include: ['src/**/*.{test,spec}.{js,ts}'],
+		environment: 'jsdom',
+		setupFiles: ['./vitest-setup.ts']
+	}
+});