Unable to use URI.resolve from root if joined path is absolute

[colin-grant-work]

Bug Description:

Calling something like URI.fromFilePath('/').resolve('/path/with/initial/separator') fails with an error If a URI does not contain an authority component, then the path cannot begin with two slash characters ("//"). (Note that URI.resolve behaves like Node's path.join, not Node's path.resolve, which is URI.resolveToAbsolute).

Originally addressed in #15331 but that fix caused problems in webviews. See #15416 and #15420.

Steps to Reproduce:

1. URI.fromFilePath('/initial/segments/').resolve('/path/with/initial/separator')
2. Observe no error.
3. URI.fromFilePath('/').resolve('/path/with/initial/separator')
4. Observe an error.

Additional Information

• Operating System: Mac OS Sequoia
• Theia Version: