Importing global PHP functions

### Description

Hello!

Right now only functions from other namespaces can be imported (and `count` for some reason) with the `use function` directive. All other display the `cannot be resolved` in the `Fix imports in Current Namespace` window.

### Use case/motivation

I have found the `use function` to be far more elegant than the prefixed syntax, but I have to do it manually. Being able to import them automatically like the class names would save a lot of tedious minor work.

### Related issues

_No response_

### Are you willing to submit a pull request?

No

### Code of Conduct

Yes

Start by locating the PHP implementation behind the “Fix imports in Current Namespace” window and compare its handling of namespaced functions with the existing `count` special case. Done means global PHP functions are recognized and offered as `use function` imports, with the existing import-fix behavior preserved.

Written by the indexing model from the issue text.