Add a migrator for plain-CSS `if()`

[nex3]

See sass/sass#3886. The proposal as currently written involves removing Sass's if() function and syntax and adding a sass() condition to the plain-CSS if() syntax instead. A migrator for this would encompass two changes:

1. Converting direct calls to the Sass if() function into calls to the plain-CSS if(). For example,

   if(
     map.has-key($tokens, $token),
     map.get($tokens, $token),
     $fallback
   )

   would become

   if(
     sass(map.has-key($tokens, $token)): map.get($tokens, $token);
     else: $fallback;
   )

   For the special case where the else branch returns null, it can be omitted entirely.

2. Converting references to if as a first-class function to references to a new custom function. For example,

   $if: meta.get-function("if");

   would become:

   @function -if($condition, $if-true, $if-false) {
     @return if(sass($condition): $if-true; else: $if-false);
   }