Pattern completions without `:` could also add it

[FMorschel]

Pattern completions without : could also add : TypeName , so we can simply write down the new variable name. Currently, it simply ends the current word but does nothing. Since we already know the user will need the type or the var/final` keyword, we can handle that to make this easier.

Repro:

void f(C c) {
  if (c case C(gett))
}

abstract class C {
  int get getter;
}

CC @bwilkerson @DanTup @srawlins