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: https://github.com/Chatterino/chatterino2/blob/694d53ad20d853e0d85e7c6a0a4ed57c9ee7e7ef/.github/workflows/build.yml#L34-L51