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_trivia/src/lib.rs

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