Add txtpb support

[hansfn]

In commit bc0c05d textproto support was added. The recommended file extension is .txtpb and it should be supported.

It might already work, but since there is no release after the commit, I wasn't able to test.

[hansfn]

It seems Clang format itself only supports / understands the .textproto file extension. Making VS code treat .txtpb as textproto language is easy, of course, using "files.associations".

A work-around is to use "Assume filename" in the extension setting, but then you force one Clang format language for all files - which is not what you want if your .clang-format file supports many languages. It seems the "Assume filename" setting needs to be per language in the extension settings.

Added: It seems Clang format recently started to support .txtbp as extension - but not in 18.