[FrameworkBundle] Fix MicroKernelTrait for php 8

[derrabus]

Q	A
Branch?	5.1
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	#36872
License	MIT
Doc PR	N/A

This PR fixes several php 8 related issues with MicroKernelTrait.

• Anonymous microkernel classes were not handled properly. I've added a test case to cover this scenario and fixed the issues.
• As part of the upgrade path, the trait logic parsed TypeErrors raised by php. That code broke because the wording of those errors has changed. I've replaced that logic with a hopefully less brittle reflection-based approach.
• In order to fix compatibility issues, @nicolas-grekas has already commented out the two abstract methods of the trait. If someone forgets to implement them, they would've ran into rather cryptic errors. I've added tests for these scenarios as well and introduced user-friendly exceptions.

I've noticed that implementing an old-style configureContainer() does not raise any deprecation. Is that on purpose? I really think that we should deprecate the old way, so we can re-add the abstract methods again in Symfony 6.

[nicolas-grekas]

Nice improvements thanks, just some nitpicking.

I've noticed that implementing an old-style configureContainer() does not raise any deprecation. Is that on purpose? I really think that we should deprecate the old way, so we can re-add the abstract methods again in Symfony 6.

Yes, that's on purpose: my opinion on this is that the is a nice extension point to easily allow extensibility of the DSL. We're fine is ppl want to use any variant of it (by taking inspiration of the one we added in 5.1).

The previous DSL is still fine, it's not deprecated and shouldn't be to me.

[derrabus]

Ladies and gentlemen, attached to this PR, you can witness the first green php 8 build of the 5.1 branch. 🎉

[nicolas-grekas]

Thank you @derrabus.

[nicolas-grekas]

And now master \o/ Thanks for your hard work @derrabus!!!