Support + and - for delegate combine and remove

Today in VB if you want to combine delegates or remove them you have to use shared methods on the delegate type, which are also loosely typed. We could support + and - like C# for combining two delegate invocation lists or removing a delegate from the invocation list of another. `Option Strict` users have requested this for a while.

Note: This proposal is for combining **delegates** not for using += and -= for event handler registration and removal. VB already has AddHandler and RemoveHandler for this.