Lint with Ultracite's Oxlint presets

Extend "ultracite/oxlint/core" and "ultracite/oxlint/vitest" in the
lint configuration and fix what the automatic fixes left behind.

Functions that return a value on some paths now return null on the
others instead of falling off the end, so restoreCache, matchVersion
and the .ocamlformat reader say "nothing found" explicitly. Conditions
on optional strings compare against undefined and the empty string,
which keeps the empty string treated as absent, as before.

The supported architectures and platforms move from switch statements
into lookup tables, and the error messages list them from there. The
exhaustiveness check would otherwise want a case for every value that
Node.js defines.

Loops that must run in order, such as adding repositories by
priority, keep their await and say why in a disable comment. So does
the Actions cache mock, which mirrors an asynchronous API without
awaiting anything. Tests sit in describe blocks, regular expressions
use the Unicode flag and named groups, and node:path is a default
import.

Author: smorimoto