Fix CI by migrating to Node's built-in test runner

[hyperupcall]

AI Notice: I used AI to do the code changes for the migration.

This PR aims to fix issues with CI in #52 by migrating away from Jest. Some facts:

• ESModules: v12.17.0 drops the --experimental-modules flag and it becomes stable in v12.22.0.
• Node's Test Runner: Stabilizes in v20.0.0. The --watch flag stabilizes in v22.0.0. Engines field was kept at v20 in this PR since it's optional.

Here's the output on my machine:

$ npm run test

> code-owner-self-merge@1.0.0 test
> node --test

✔ determine who owns a set of files (11.554664ms)
✔ real world (1.38568ms)
✔ real world 2 (1.264941ms)
✔ real world with labels (1.246522ms)
✔ deciding if someone has access to merge (2.901409ms)
▶ githubLoginIsInCodeowners
  ✔ allows folks found in the codeowners (1.496068ms)
  ✔ ignores case (0.876618ms)
  ✔ denies other accounts (1.005145ms)
  ✔ denies subsets of existing accounts (0.805349ms)
✔ githubLoginIsInCodeowners (5.029728ms)
▶ hasValidLgtmSubstring
  ✔ allows lgtm (0.390384ms)
  ✔ denies lgtm but (0.107018ms)
  ✔ denies lgtm but with comma (0.068499ms)
  ✔ denies lgtm in double quotes (0.067529ms)
  ✔ denies lgtm in single quotes (0.069619ms)
  ✔ denies lgtm in inline code blocks (0.060729ms)
✔ hasValidLgtmSubstring (1.038765ms)
ℹ tests 15
ℹ suites 2
ℹ pass 15
ℹ fail 0
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 152.327036

Lastly, I also used pinact to update the versions of the actions in GitHub actions.