Add Ruby language injections

[BuonOmo]

This adds support for Ruby heredoc's syntax highlighting. The injection was directly taken from the tree-sitter documentation.

It is quite simple, but has the drawback of only showing highlighting once the heredoc is fully written and next line is started. This is due to the fact that we use the last line of the heredoc to determine the language. As using the first one would require some cleaning up that we cannot do trivially. (I might have not fully understood the behaviour of the #match? predicate, which could help us)

Fixes #4473

Release Notes:

• Added Ruby language injections (#4473).

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[BuonOmo]

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[maxbrunsfeld]

Thanks @BuonOmo !