[12.x] add allDirectories() method to Filesytem

[browner12]

we already have files(), allFiles(), and directories(), so this method completes the set and allows us to recursively get all directories within a given path.

both files() and allFiles() return an array of SplFileInfo objects, while directories() maps the results to the path name and returns an array of strings. I was torn between wanting to be consistent with the directories() method here, but also wanting to return the full object to allow the calling code to decide what it needed. Ideally all 4 methods would return an array of SplFileInfo objects, but that would be a BC. I landed on providing the objects back, but open to suggestions.

[shaedrich]

adding folders(): SplFileInfo[] additionally to directories(): string[] (and allFolders(): SplFileInfo[] vs allDirectories(): string[] respectively) would probably confuse people—what do you think?

[jmarble]

I agree with @shaedrich -- I think allDirectories() should return an array of strings just like directories()

[taylorotwell]

Yeah probably should return strings.