draft: Add snippets

[gadenbuie]

@georgestagg @wch In returning to #153, I wondered if the feature I'm actually wanting is snippets.

I took a stab at defining language-specific shiny snippets for R and Python, using the same snippets we use in the Shiny VS Code extension...

...and it almost works. I have the sense I'm fairly close (drawing mainly from this example), but the snippets aren't appearing in the editor. I'm hoping one of you might be able to quickly spot the issue.

[georgestagg]

I've made a commit that gets things working for me. With this change, typing shiny in the editor offers the snippets as autocomplete options.

We could think about populating other properties like displayLabel⁠ alongside the label, so as to make the snippets clearer in the autocomplete list. Providing a type would give the entries an icon, for example. We could also think about setting boost to some number so that snippets float to the top.

[georgestagg]

I'm not sure why, but removing this definitely was required to get snippets working. I guess it's replacing the autocompletion extension setup by the file above.

In a quick test, removing this did not seem to do any harm and context aware completion still seemed to work. Still, we should try and work out why this was originally here and if it's still strictly necessary - though it doesn't seem to be.