Release plugins alpha

[Mm2PL]

Adds a scary label and renames plugins from EXPERIMENTAL to alpha and enables them on normal builds. Things to do around merge:

• Rename the plugins label
• I'd really like someone to look more carefully at the plugins code. I hope I didn't make any security mistakes in that code. Anyone is more than welcome to submit feedback about this code here.

[Nerixyz]

Btw, plugins are already enabled on the Ubuntu builds:

chatterino2/.github/workflows/build.yml

Lines 34 to 51 in 694d53a

	- os: ubuntu-20.04
	container: ghcr.io/chatterino/chatterino2-build-ubuntu-20.04:latest
	qt-version: 6.6.1
	force-lto: false
	plugins: true
	skip-artifact: false
	skip-crashpad: false
	build-appimage: false
	build-deb: true
	- os: ubuntu-22.04
	container: ghcr.io/chatterino/chatterino2-build-ubuntu-22.04:latest
	qt-version: 6.6.1
	force-lto: false
	plugins: true
	skip-artifact: false
	skip-crashpad: false
	build-appimage: true
	build-deb: true

[pajlada]

Leave CHATTERINO_PLUGINS off by default in the CMakeLists.txt, but enable it in our CI, move the changelog up (to the new unversioned category after merging), then I'm good to have this merged