Switch to using setTextDocumentLanguage instead of adding to files.associations

[sean-mcmanus]

The Septemember 2018 release added vscode.languages.setTextDocumentLanguage, so our extension can try using that instead of adding files to files.associations.

The previous thread is #722 .

[sean-mcmanus]

Actually, this is problematic, because setTextDocumentLanguage is not persistent like files.associations is...

[bobbrow]

@sean-mcmanus did you try this and verify that it does not persist? We may need to re-engage with the VS Code team on this and come up with a solution that works for us.

[bobbrow]

The VS Code issue on this: microsoft/vscode#78968

[sean-mcmanus]

I think this needs to be spec'd out. The set document language doesn't persist. We could persist it in a "hidden" way, but then users wouldn't have any ability to set those settings. Maybe we could add a setting for users who strong don't want the files.associations in settings.json, and maybe we could change the "hidden" setting after the user manually changes the language via the status bar (not sure).