Rollup merge of rust-lang#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

.gitattributes

@@ -5,6 +5,7 @@
 *.h rust
 *.rs rust diff=rust
 *.fixed linguist-language=Rust
+*.pp linguist-language=Rust
 *.mir linguist-language=Rust
 src/etc/installer/gfx/* binary
 src/vendor/** -text