Unify a way how to load stubs for extensions

[alies-dev]

Idea: do not use extension_loaded() function to determinate loaded extensions. Instead, Psalm already has 2 ways of doing this:

1. composer.json (declare ext in require block)
2. using config: enableExtentions: https://psalm.dev/docs/running_psalm/configuration/#enableextensions

Note, it may potentially change live output from code snippets on psalm.dev. To fix it, please use one of 2 methods from a list above

Why?

1. Reduce entopy
2. Provide potential contributors a clear way how to support extensions vis stubs

Linked discussion

#8879

[alies-dev]

replaced by #8885