create links for values of format: uri-reference

[twelvelabs]

This PR adds logic to findLinks to create links for relative path names when the associated JSON schema is configured with format: uri-reference.

Fixes: microsoft/vscode#128971 Unblocks: redhat-developer/vscode-yaml#559

A few notes:

• I tried to make the change as backwards compatible as possible by adding a new, optional schemaService param to findLinks.

• This was important because the YAML language server delegates to findLinks, and I'm primarily interested in getting this functionality working over there.

• I assumed that the fileExists check would be a requirement (i.e. links for non existent files would be a poor UX). It's only being ran for properties w/ format: uri-reference, so it shouldn't be much overhead, but I'm happy to remove if you disagree.

[twelvelabs]

@microsoft-github-policy-service agree

[twelvelabs]

ping @aeschli