[vscode][proposed] documentPaste proposed API updates for 1.79

[rschnekenbu]

documentPaste proposed API has evolved between 1.78 and 1.79:

• DocumentPasteEdit now has several required properties: id, label and optional priority. Constructor has also evolved to reflect these changes.
• DocumentPasteProviderMetadata has a new optional Property (copyMimeTypes)
• DocumentPasteEditProvider#providerDocumentPasteEdits() is now optional.

This proposed API is used by several builtins (ipynb & markdown-language-feature)

[JonasHelming]

@msujew When do you plan to work on this? Would be nice to have this and #12632
for the upcoming community release

[msujew]

@JonasHelming I should have some time to work on both of these issues this week.