Replace PHP_Codesniffer and Psalm with Mago

[gsteel]

The baselines here are pretty large:

• SA: 428 issues: 142 errors, 256 warnings, 30 help messages
• Lint: 172 issues: 49 errors, 64 warnings, 59 help messages

This is a relatively low-effort PR to illustrate what's involved to make the switch.

Bear in mind the the psalm baseline was nearly empty here.

For comparison, Mago's lint, analyse and format runs each take a second, compared to Psalm at 35 - 40 seconds on this codebase.

[froschdesign]

@gsteel
Shall we discuss this at the next meeting? This pull request would be a great example of that.

[gsteel]

@froschdesign
Just writing a patch for tonights meeting now. That's what this is for 👍