Partiallyfixes#46568.

Whenever completions offer a method snippet, the parameter names and types, and the return type are snippet placeholders, as described in#46370. And, if the method has a body, there is a tabstop in the empty body.

We are assuming that, in most cases, the person using the method snippet won't want to change the method signature, and will instead just want to fill in the body with the implementation of the method.
For that reason, the tabstop in the body is thefirst one, so when the method snippet is inserted, the cursor is in the body, like so:

classBarimplementsFoo{bar(a:string,b:number,c:string,d:number,e:string):void{|// cursor starts here}}

This PR adds a second tabstop, namely$0, right next to the first tabstop, in the body of the method.$0 is special, in that it is the last tabstop. This means that the following flow is possible:

classBarimplementsFoo{bar(a:string,b:number,c:string,d:number,e:string):void{|// cursor starts here}}

after pressing tab a few times, navigating through the method signature:

classBarimplementsFoo{bar(a:string,b:number,c:string|,d:number,e:string):void{}}

after pressing tab one last time, after going through (and possibly editing) the parameters and return type:

classBarimplementsFoo{bar(a:string,b:number,c:|string,d:number,e:string):void{|// cursor ends here}}