Add MethodDescriptor to TypeScript definitions

[smnbbrv]

A followup of #865 (comment)

The TypeScript definitions do not have the MethodDescriptor exported.

With bypassing the typings

declare var require;

console.log(require('grpc-web').MethodDescriptor);

I can read it in TypeScript.

Looks like grpc-web's TypeScript mode still uses the deprecated MethodInfo instead and should be updated as well.