Rollup merge of #139106 - ShE3py:pp-highlighting, r=compiler-errors

Mark .pp files as Rust

Pretty-printing tests generate `.pp` files, but GitHub classify and highlight them as Pascal:
[https://github.com/search?q=repo:rust-lang/rust+path:*.pp&type=code](https://github.com/search?q=repo%3Arust-lang%2Frust+path%3A*.pp&type=code)

``@rustbot`` label +A-meta +A-testsuite

Author: matthiaskrgr