support adapter for IDocument in LSPEclipseUtils

[raghucssit]

org.eclipse.lsp4e.LSPEclipseUtils.getFile(IDocument) and org.eclipse.lsp4e.LSPEclipseUtils.toUri(IDocument) uses the eclipse buffer allocated by buffer manager to IDocument to determine the java.netURI and org.eclipse.core.resources.IFile.

Some framework like Xtext does not connect their document to buffer manager. So LSPEclipseUtils returns null for URI and IFile in such cases.
So we can try to adapt the IDocument to URI and IFile. Off course implementors of IDocument shold adapt themselves for this feature to work.

I have created a PR to support adaption of XtextDocument but that is not precondition for this PR.
eclipse-xtext/xtext#3614

More background:
Github copilot uses LSPEclipseUtils to derive URI and IFile. Which is currently failing for XtextDocuments and lacking the copilot completion support for DSL editors.
microsoft/copilot-eclipse-feedback#44

[iloveeclipse]

@rubenporras : thanks for merging #1501.

Could you please:

1. Mark this ticket as fixed by Adapt document to IFile when unable to retrieve file from buffer manager #1501
2. Assign it to @raghucssit
3. If you use milestones, mark for which milestone / release it is planned.

Thanks!

[iloveeclipse]

@rubenporras : currently the enhancement only works via (invisible to user) preference.

Would you mind if we would provide some visible preference option somewhere in LSP4E, something like "Enable support for Xtext like editors" or "Enable document retrieval fallback via resources API"?