Defend against directories without trailing slashes

[precup]

Fixes #103992

FileSystemDock::create_directory can get called with a base_dir that don't have a trailing slash in a couple ways. It seemed like a footgun for it to require a trailing slash, so I fixed the issue by making it tolerant to either formatting.

After that, I audited similar usages of trim combined with paths in the rest of the codebase since this seems like an easy mistake to make, and the editor_file_system change addresses the only one that seemed similarly easy to make a mistake with and is a cheap enough addition it seemed reasonable to add defensively.

This largely occurs because String::get_base_dir returns a result without a trailing slash. Since this is also a base_dir, it seems like it should accept things straight from String::get_base_dir.