Add snippet completions for methods

[mjbvz]

From https://github.com/microsoft/TypeScript/issues/47044

For the code:

class Base {
  foo() { }
}

class Sub extends Base {
  
}

In TS 6.0 triggering suggestions in Sub and selecting foo would fill in:

  foo(): void {
    
  }

In Ts-go, we instead just fill in foo