Add placeholders to callable completions

[erictraut]

I'm moving this feature request from pyright to pylance. It was originally submitted by @leungbk here microsoft/pyright#924.

It would be nice if snippets were given in completions. For example, with the insertion point at [-] in

def factorial(n: int) -> int:
    return 1 if n == 0 else n * fa[-]

one of the completions could be, using snippet syntax,

factorial${1:(${2:n: int})}

Type-inferencing could be useful here to let us automatically drop the outer pair of braces depending on context.

[jakebailey]

This is technically #37 + placeholders for parameters. We've committed to the former, but the latter has a pretty poor UX in VS Code, IMO. Other LSs either don't implement placeholders at all, or at least make it toggleable.

[savannahostrowski]

This issue has been open for nearly a year and has not received enough upvotes to prioritize the work. Will reopen if we see this becoming a theme in user issues.