gh-104169: Refactor tokenizer into lexer and wrappers (#110684)

* The lexer, which include the actual lexeme producing logic, goes into
  the `lexer` directory.
* The wrappers, one wrapper per input mode (file, string, utf-8, and
  readline), go into the `tokenizer` directory and include logic for
  creating a lexer instance and managing the buffer for different modes.
---------

Co-authored-by: Pablo Galindo <pablogsal@gmail.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>

Parser/peg_api.c

@@ -1,6 +1,5 @@
 #include "Python.h"
 
-#include "tokenizer.h"
 #include "pegen.h"
 
 mod_ty