refactor!: migrate away from bun

- migrate package management to pnpm
- migrate test suite to vitest
- also remove Anify integration

src/libs/changeStringCase.spec.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it } from "bun:test";
+import { describe, expect, it } from "vitest";
 
 import { Case, changeStringCase } from "./changeStringCase";