This commit makes both APIs more consistent by doing the following:
- Remove the `PyPegen_CodeObjectFrom*` functions, which weren't used
and will probably not be needed. Functions like `Py_CompileStringObject`
can be used instead.
- Include a `const char *filename` parameter in `PyPegen_ASTFromString`.
- Rename `PyPegen_ASTFromFile` to `PyPegen_ASTFromFilename`, because
its signature is not the same with `PyParser_ASTFromFile`.
This commit also allows to pass flags to the new parser in all interfaces and fixes a bug in the parser generator that was causing to inline rules with actions, making them disappear.