Experiment with trailing comma insertion text for object literal completions

[DanielRosenwasser]

When I'm adding a new property in the middle of an object literal, there's nothing I love more than getting an immediate parse error and then having to add a comma after the body of a nested object literal/method declaration. It'd be nice to see if we can smooth out the experience here. Maybe we can have comma insertion text after these completions (provided the experience doesn't feel annoying).

[ghost]

I think this is blocked by microsoft/vscode#39893, since the comma we add would be on the same line as the completion.
There's also the problem that if you're typing multiple entries in an object literal normaly, you'd get extra commas:

Typing const x = { a: 0, b: 1 }:

• Start:
  • const x = { |
• Complete a:
  • const x = { a: |,
• Type 0:
  • const x = { a: 0|,
• Type , :
  • const x = { a: 0, |,
• Complete b:
  • const x = { a: 0, b: |,,
• Type 1 }:
  • const x = { a: 0, b: 1 }|,,