Metadata handling improvements

[jonsequitur]

These changes fix the following issues:

• New code cells in F#- (or any non-C# language-) defaulted notebook are missing metadata, are treated as C# #3544
• Polyglot Notebook: [DevExE2E] When saving the newly created .dib/ipynb file, the cell execution results will be disappeared and these files still in unsaved status.  #3576 for .ipynb files. I'm still looking for the root cause of this same issue for .dib files.

This is a work in progress and additional PRs will continue to investigate and address some of the FIX comments present here but this is likely worth merging in order to do a preview release and validate the changes against a larger number of existing notebook documents than the tests likely account for. I anticipate that some of these changes will break backwards compatibility for metadata variations that date back to before the VS Code notebook APIs were finalized. I don't believe it's practical to support those forever, and any notebooks that are broken as a result will at this point be far fewer than the impact of these bugs on newer and newly-created notebooks.