Incorporate the `ExamplePlugin` target into the `MacroExamples` target

[Matejkob]

          I'm wondering if we still need the [`ExamplePlugin` target in the `Examples` package](https://github.com/apple/swift-syntax/tree/main/Examples/Sources/ExamplePlugin) - was it mainly used for lit tests, and is it now a bit redundant with `MacroExamples`?

Originally posted by @Matejkob in #2160 (review)

Since the last lit test has been removed we don't need a separated ExamplePlugin target so we can merge macros into MacroExamples.

[Matejkob]

I'm on it 😇

[ahoppen]

Tracked in Apple’s issue tracker as rdar://115120727

[Matejkob]

Blocked by #2163