Two build fixes found while getting the build working on FreeBSD.
1. **Remove hardcoded `#!/bin/bash` paths and use /usr/bin/env instead
/**
2. **env_remove("YARN_BINARY")**
Tested on FreeBSD 16.0-CURRENT amd64.
A second series adds FreeBSD support with `platform` variants,
environment overrides, PyQt6 handling and docs.
Closes#5301
Earlier today I pushed a change that split this code up into multiple
repos, but that has proved to complicate things too much. So we're
back to a single repo, except the individual submodules are better
separated than they were before.
The README files need updating again; I will push them out soon.
Aside from splitting out the different modules, the sound code has
moved from from anki to aqt.
