rust-analyzer/meson: activate rust-analyzer with meson buildsystems

Merged Günther Wagner requested to merge gwagner/gnome-builder:rust-analyzer into master

With the notification patch i basically only allow cargo buildsystems running rust-analyzer. This patch should fix this again with a "sophisticated" approach. Every meson buildsystem can define the languages in the project declaration (unfortunately its optional).

This information can be used to check for rust as part of the buildsystem and activate in this case rust-analyzer.

I am open for better approaches. Vanadiae mentioned maybe we can guard via EditorPage in a similar way the html-preview plugin guards against. In a Pipeline Addin i found no simple way to read out this information.

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