plots: quote and resolve paths properly

[skshetry]

This commit fixes 3 things:

1. The path printed on the terminal is properly is quoted. This was
   problematic when the output is piped to xdg-open if the path contained
   spaces or quotes or special characters.

$ dvc plots show -o 'plots with spaces.html' | xargs xdg-open
xdg-open: unexpected argument 'with'
Try 'xdg-open --help' for more information.

2. The path is resolved. This is not an issue at all but makes the
   output look nicer.

$ dvc plots show -o '../plots.html' --open
file:///home/saugat/repos/iterative/example-get-started/../plots.html

3. The path with which the browser is opened with uses relative path.
   This is done to solve issue in WSL, that the absolute path is not
   accessible outside the WSL for host programs as the linux filesystem
   is available on /wsl$ path.

$ dvc plots show --open
file:///home/saugat/example-get-started/plots.html
Start : This command cannot be run due to the error: The system cannot find the file specified.
At line:1 char:1
+ Start "/home/saugat/example-get-started/plots.html"
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [Start-Process], InvalidOperationException
    + FullyQualifiedErrorId : InvalidOperationException,Microsoft.PowerShell.Commands.StartProcessCommand

4. On Windows, the file-uri was being constructed using platform-dependent os.path.join. This has been changed to use as_uri() from pathlib.Path.

With the fix to use relative path as argument to webbrowser.open, I expect WSL to be able to work from outside (tested) as well as within inside if needed (didnot test at all). This does work on Linux.

See #5584 (comment)

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[skshetry]

Need to do a QA on Windows for the file-uri changes. Previously, the file-uri where being constructed through platform-specific paths, which means there were \\ in them. Needs to be checked if those were valid before.

[skshetry]

Was not able to test out on windows, I was curious if it worked before there, but probably it didn't. 🤞🏼