egui_extras: percent-decode file:// URIs in the file loader

[Lfan-ke]

FileLoader strips the file:// prefix but does not percent-decode the path,
so a file:// URI with an escape such as %20 fails to resolve on disk (the
function's own doc noted "percent un-encoding are not handled").

convert_uri_to_path now decodes %XX escapes (e.g. %20 -> space) after
stripping the scheme, using a small inline decoder so no dependency is added.
Invalid or truncated escapes are left as-is, and UTF-8 multibyte sequences round
-trip. Added %20 cases to the existing check_convert_uri_to_path test for
both the Windows and non-Windows paths.

cargo test, clippy, and fmt pass for egui_extras.

Closes #8288

[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/8342-fix/file-loader-percent-decode

View snapshot changes at kitdiff