Support for converting between unix-style `Path` and `Url` on Windows

Often in tests I'll use only unix style paths (ex. /data/test.txt) because it works with Path on both Windows and Unix.

The problem I have though, is the Url::from_file_path(path) and url.to_file_path() both error on Windows for these kind of paths and specifiers in this crate even though they can be represented just fine as a std::path::Path. For example:

let path = PathBuf::from("/testing");
// Unix: Ok(_)
// Windows: Err(())
println!("{:?}", Url::from_file_path(&path));

let url = Url::parse("file:///testing").unwrap();
// Unix: Ok("/testing")
// Windows: Err(())
println!("{:?}", url.to_file_path());

Is there a reason these aren't supported on Windows? Would you accept a PR that adds support for this?

Start by tracing Url::from_file_path and Url::to_file_path on Windows, comparing their handling of unix-style paths with std::path::Path. Confirm the intended behavior for the examples in this issue and add coverage for both conversions; done means those examples no longer return Err(()) on Windows without breaking existing path handling.