Porting category: Add OS name as subcategory where it can be inferred

[Rémi Verschelde]

For issues with topic:porting label, we currently add the "Porting:" prefix, which is good.

I'm used to also specifying the platform if it's platform-specific (i.e. only one platform: label), e.g. "Porting: Linux: Blabla PR title" if the labels are topic:porting platform:linuxbsd). We should remember to also handle stripping already existing platform prefixes when they exist (e.g. a PR named "Android: Blabla").

If there are more than one platform: labels, then we should fall back to just "Porting:" as it's a cross-platform change.

For topic:linuxbsd, I would suggest using "Linux:" as prefix. It's worth noting that we have PRs using "X11:" or "[X11]" as prefix in their PR title, and sometimes it's indeed for X11 specific stuff, and sometimes just a shorthand for the Linux/X11 platform name. I think it's fine if those end up as "Porting: Linux: X11:" even if the change isn't specific to X11 code, it's not a big deal (and we can fix those by renaming the PRs when we spot them). Likely we should match existing prefixes like "LinuxBSD:" etc. and harmonize it to just "Linux:".

#8 may be a prerequisite to properly handle cases such as this one:

Porting: [iOS] Fix orientation change in runtime. ([GH-78132](https://github.com/godotengine/godot/pull/78132)).