Add unit tests

[m4rvr]

This PR adds some basic unit test setup with jest + esbuild-jest. Currently, only a few utils are tested.
Also, the test script/command needs to be run manually.

[frandiox]

@MarvinRudolph Thanks! As mentioned on Twitter DMs, we are going to use uvu instead of Jest. The other PR was already merged so we should adapt this PR to the new structure.

@m3hari How should we handle unit tests like the ones in this PR? Also in test/specs or somewhere else? 🤔

[m3hari]

Apologies for the late reply, I think unit tests are better placed next to the file to be tested. uvu should find it with minor changes on the test command. Here is the docs for uvu test file matching

[frandiox]

@m3hari Thanks!

@MarvinRudolph Do you want to transform this to use uvu? For example, if you want to test utils/html.ts then the spec can be located next to it, in utils/html.unit.ts or utils/html.spec.ts.