"CodeQL: Open Referenced File" command fails on Windows #970

Describe the bug
The command CodeQL: Open Referenced File fails on Windows due to an erroneous leading / on the file path pointing to the ql file. This is likely due to a bad conversion between URI and file path.

Version
Extension v1.5.5
CodeQL CLI v2.6.2
Version: 1.60.2

To reproduce
Run the CodeQL: Open Referenced File command on a windows machine.

Expected behavior
The referenced ql file should open in the editor without error.

Additional context
This issue was originally part of #921 , sequestered due to its complexity.