gh-113317: Finish splitting Argument Clinic into sub-files (#117513)

Add libclinic.parser module and move the following classes and
functions there:

* Parser
* PythonParser
* create_parser_namespace()

Add libclinic.dsl_parser module and move the following classes,
functions and variables there:

* ConverterArgs
* DSLParser
* FunctionNames
* IndentStack
* ParamState
* StateKeeper
* eval_ast_expr()
* unsupported_special_methods

Add libclinic.app module and move the Clinic class there.

Add libclinic.cli module and move the following functions there:

* create_cli()
* main()
* parse_file()
* run_clinic()