Add PHPStan and other quality checks

[Alkarex]

Similar to FreshRSS core
Contributes to #184
Uses FreshRSS/FreshRSS#5901

TODO:

• Check compatibility with PHP 7.4 - PHP 8.3

[Alkarex]

Only lacking a bit of work on:

• ./xExtension-ImageProxy/configure.phtml
• ./xExtension-ImageProxy/extension.php

Any takers please 🙏🏻?
Maybe @Frenzie or @ColonelMoutarde?

This PR can be also merged as-is after a bit of testing.
Anyone to give it a try? @lapineige or @math-GH maybe?

[Alkarex]

I thought there was an issue (maybe by @math-GH ) somewhere about doing this PR, but I cannot find it at the moment... (Hints welcome)

[lapineige]

I've no time to test it, sorry.

[Frenzie]

@Alkarex Probably not for a week, slightly depending on the weather.

[math-GH]

Sorry, I am late to the party.
In my first check I saw an PHP error with 1.22.2-dev about 10 commits behind. I mean: Does this PR works fine with the current 1.22.1? The users have 1.22.1 installed and add extensions from the repository. Sure that the current extensions (that includes this PR) work fine?

[Alkarex]

No, there is no possible path of update without also making a new FreshRSS release, so we need to issue 1.23.0 as soon as possible.
It also indicates the need to have some version ranges for compatibility in the metadata. FreshRSS/FreshRSS#5903
Help very much wanted in finalising the work on the extension updates (tests in particular, before the release of 1.23.0)

xExtension-ImageProxy is also in need of a caring hand (maybe @ColonelMoutarde while @Frenzie is away)

[Alkarex]

What lacks most urgently is testing the compatibility with older third-party extensions, which have not been updated.
Edit: this looks ok, at a quick glance (edge, with PHP 7.4, 8.2, 8.3) - more tests welcome

[Alkarex]

Related: FreshRSS/FreshRSS#5837