Provide a more intuitive mechanism to add an extension method for a class to a module in the IDE

Currently to add an extension method in the IDE, the user has to create/select a module, insert a method, then type Extends s As String manually into the parameters field. That means the current UI still leaks the language syntax and is not the IDE-first experience we want.

There should be a new item in the Insert menu and the Add button and in appropriate contextual menus (i.e. modules) to "Add Extension Method". This should present a new member editor that has a field for the class to extend, a text field for the parameters (if any) with syntax highlighting like in the regular method editor and a return type (if any).