Add protobuf support

[MordFustang21]

Release Notes:

• Added protobuf syntax highlighting (#5160).

[SomeoneToIgnore]

Looks much better now, thank you 🚀

I think we would need to add a bit more:

• optional keyword highlights
• oneof variant highlighting also asks for more colors
• outline.scm so a person could navigate over the next with cmd-shift-o menu + enable the gutter showing regions on top
• ? maybe, add some slight color change to the field names, so they do not mix with the unknown identifiers
• and a small formatting nit the CI complained about needs to be fixed

[MordFustang21]

I've addressed most of the above comments. There are still some outstanding issues though that I'm not quite sure how to tackle.

1. oneof variant highlighting also asks for more colors
2. ? maybe, add some slight color change to the field names, so they do not mix with the unknown identifiers

@mikayla-maki I added an outline file with some queries that felt most relevant. Happy to pair on it sometime if you feel there's more to add.

[mikayla-maki]

Thank you for this PR! :D

[SomeoneToIgnore]

Let's add a new doc entry into https://github.com/zed-industries/zed/tree/main/docs/src/languages list and merge this 🎉

[MordFustang21]

Added doc file