config: File capabilities lookup is broken for Windows paths

[charlieegan3]

unexpected error from Lookup: failed to parse URL 
'file://D:\a\regal\regal\internal\capabilities\testdata\capabilities.json': parse 
"file://D:\\a\\regal\\regal\\internal\\capabilities\\testdata\\capabilities.json": invalid port 
":\\a\\regal\\regal\\internal\\capabilities\\testdata\\capabilities.json" after host

The current code is written expecting URIs for be in format:

file:///
       ^--note

We can use this, but it's then inconsistent with our other uses of URIs for windows paths in the LSP, for example.