Optimize get_path() in EditorFileSystemDirectory

[KoBeWi]

EditorFileSystemDirectory's get_path() works by traversing its parent directories and concatenating strings. It's horribly inefficient, but seems to be called a lot during scans.

I optimized it using PackedStringArray (the path is built in reverse order, so couldn't really use StringBuilder) and the optimized version seems to be ~150% faster. This of course depends on nesting level, I tested with 6 levels of nesting.

Alternative is adding a cached path String to each EditorFileSystemDirectory, which would be even faster, but use more memory.

[Mickeon]

This optimization is great. I just worry the new code may have become a bit difficult to follow.

[akien-mga]

The code in get_file_path is almost the same and should likely be optimized similarly.

Then I'd suggest adding a comment that explains why it's done this way, so it doesn't get refactored in the more intuitive but slower form.

[Mickeon]

Makes much more sense now. I like the code between the two methods being united like this.

The performance benefit may be even more with #92550

[akien-mga]

Thanks!