feat: add wrapping of global function with existing check

[Katalam]

• bug fix / new feature?
• feature
• BC break? yes/no
• no
• doc PR: nette/docs#???
• can be done if merged

I'm generating a lot of helper methods recently, but I have the need to wrap them into a function exist check. It is kinda duplicated if done manually, so I want it to be part of the generation itself. The code style may differ a lot from the code style of the project, feel free to adjust it

[dg]

The flag indicating that a function should be wrapped in a condition does not logically belong to the GlobalFunction class but is rather a concern of the Printer.

If you need to wrap all functions in a file this way, it can be easily handled with a single switch in Printer. However, if the wrapping needs to be selective, the implementation would naturally be more complex.