Add adapter factory support for XtextDocument (URI / IFile / IPath adapters)

[raghucssit]

Background
• LSPEclipseUtils.toUri(IDocument) tries to derive a java.net.URI / IFile from an IDocument.
• GitHub Copilot rely on LSP4E to obtain a document URI / IFile.

As this support is not there github copilot is not setting correct context for XtextDocuments and thereby not providing any copilot suggestions on DSL editors.

Here is the github copilot issue.
microsoft/copilot-for-eclipse#46