feat(extgen): make the generator idempotent and avoid touching the original source

[alexandre-daubois]

The DX is currently not optimal as we need to touch and edit the original source file. It is still in WIP, but this PR aims to improve a lot the DX of the extension generator by making it idempotent.

The proposed approach is to generate a wrapper to the original source file that will call it and then make the bindings to C. The original Go source will remain untouched and the generator can be run multiple times easily.

This also adds a header comment to all generated files (C file, C headers et al.):

package ext

// AUTOGENERATED FILE - DO NOT EDIT.
//
// This file has been automatically generated by FrankenPHP extension generator
// and should not be edited as it will be overwritten when running the
// extension generator again.
//
// You may edit the file and remove this comment if you plan to manually maintain
// this file going forward.

// #include <stdlib.h>
// #include "ext.h"
import "C"

// rest of the file...

[alexandre-daubois]

Requires #2086 to be merged first