project: Fix content not displaying when selecting a folder in Windows (#18946)

- Closes #16998

This PR resolves issues with the /file and /diagnostics commands in the
assistant panel, which previously failed to display the contents of a
directory when searching for a folder instead of using the arrow button.

- Changed the format in `project.rs` (located at
`crates/project/src/project.rs`) to use `std::path::MAIN_SEPARATOR` for
cross-platform compatibility, which resolves errors encountered on
Windows that originally used the format `format!("{}/", ...)`.


Release Notes:

- N/A