add es Proxy for calling ExtendScript

[timhaywood]

This adds a new method for calling your ExtendScript functions, (tentatively) called es.

This uses a Proxy so you can call them as methods, rather than passing in the function name as a string. It also enables using VS Code's "Go to Definition" to go to the ExtendScript implementation of the function.

const res = await es.myFunc("test");

Under the hood it uses the existing evalTS function.

Think I added it in all the right spots, made it the default in the README rather than adding a third option, but whatever you think works best!