fix(#2281): Prevent double slashes when joining URLs

[woubuc]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Docs
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[amrbashir]

Thanks for finding and fixing the bug.
Don't forget to add a change file in .changes folder

[woubuc]

• The custom format!() call has been replaced with the Url.join() function which handles concatenating and normalising URLs a lot better.
  • WindowManager.get_url() now returns a Cow<Url> to make it possible for other functions to use the Url struct directly. I've opted to use Cow to prevent cloning the URL since we just need to read it.
  • The relevant tests in manager.rs have been updated as well to reflect this change.
  • I've also changed the check for external URLs to compare against Url.scheme() instead of converting the entire Url to a string first.
• A new Error variant InvalidUrl is now used to return parse errors that arise from the url functions (e.g. when trying to load a path with invalid characters).
• I've also reduced some of the duplicate code in the WindowManager.get_url() functions, the cfg-dependant lines are now a separate function and there is only one match. I think this makes it a bit clearer what happens here. Do let me know if I've misunderstood what the cfg differences are though.

[amrbashir]

LGTM, thanks!

[woubuc]

Looks like I forgot to run Clippy locally before committing.. Should be fixed now.

[lemarier]

Thanks @woubuc !

We do appreciate a lot your contribution!
Have a great day.