lsp/completions: Snippets for built-in functions

[charlieegan3]

Provide snippet completions for OPA built-in functions.

Similar to what we are providing for some, and every:

.

Perhaps these could even be generated snippets?

[anderseknert]

While I'm not against this, I think it could be made somewhat moot once we implement signature help.

With that flow enabled, a user would first type a few letters to see completion suggestions for functions, then once they've picked one see a tooltip for each argument after ( and each , until they'd reach )... suggesting the name and type of the argument, and whatever else we might want to display there.

OTOH, snippets are nice in how you can tab between the args, and then finally tab to the next location. But since signature help is a "standard feature" of the language server protocol, I think I'd prefer to have us implement that first, and then see if we still think snippets add value on top of that 🙂