Rename ruff_python_whitespace to ruff_python_trivia (#5886)

## Summary

This crate now contains utilities for dealing with trivia more broadly:
whitespace, newlines, "simple" trivia lexing, etc. So renaming it to
reflect its increased responsibilities.

To avoid conflicts, I've also renamed `Token` and `TokenKind` to
`SimpleToken` and `SimpleTokenKind`.

crates/ruff_python_whitespace/src/lib.rs

@@ -1,9 +0,0 @@
-mod cursor;
-mod newlines;
-mod tokenizer;
-mod whitespace;
-
-pub use cursor::*;
-pub use newlines::*;
-pub use tokenizer::*;
-pub use whitespace::*;