test: add unit tests for the module (#892)

Co-authored-by: Benjamin Canac <canacb1@gmail.com>

test/vitest.d.ts

@@ -0,0 +1,10 @@
+import type { Assertion, AsymmetricMatchersContaining } from 'vitest'
+
+interface CustomMatchers<R = unknown> {
+  toBeRegExp(expected: string | RegExp): R
+}
+
+declare module 'vitest' {
+  interface Assertion<T = any> extends CustomMatchers<T> {}
+  interface AsymmetricMatchersContaining extends CustomMatchers {}
+}