convert faucet-client to ESM

[dynst]

https://antfu.me/posts/move-on-to-esm-only
https://joyeecheung.github.io/blog/2024/03/18/require-esm-in-node-js/

Since there's no point maintaining a CJS build, just completely convert the smallest package as a proof of concept for #1004.

https://www.typescriptlang.org/docs/handbook/modules/reference.html#the-module-compiler-option

Since the build uses tsc directly instead of a build system like Vite+Rollup or a bundler that produces a single file like esbuild, explicit file extensions are required in imports (TypeScript won't add omitted file extensions and transpiles .ts files to .js files 1:1).