Add composer.json to allow installing with PIE (PHP Installer for Extensions)

[JanTvrdik]

• https://thephp.foundation/blog/2024/11/19/pie-pre-release/
• https://github.com/php/pie/blob/main/docs/extension-maintainers.md

[NoiseByNorthwest]

I'm open to an external contribution, the only thing I can say is that the composer.json should start with:

{
    "name": "NoiseByNorthwest/php-spx",
    "type": "php-ext",
    "license": "GPL-3.0",
    "description": "A simple & straight-to-the-point PHP profiling extension with its built-in web UI",
    "require": {
        "php": ">= 5.4.0"
    },
    "php-ext": {
        "extension-name": "SPX",