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

[mikayla-maki]

Thank you so much for adding this language! I've been meaning to do this for ages 😅.

I added a few small things, and agree with my colleagues about the alphabetizing, highlighting, and outline file. If you'd like to drop by our channels or fireside hacks I'd be happy to help build the outline file. It would be great to get this one merged in :D

[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

[SomeoneToIgnore]

I think it's still a bit odd to have the fields as defaults, but since I have no good proposal on how to capture it, let's move on.

Thank you for taking a stab at this, now Zed repo is better highlighted.