bpo-40334: Make the PyPegen* and PyParser* APIs more consistent (GH-19839)

bpo-40334: Make the PyPegen* and PyParser* APIs more consistent (GH-19839)

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`.