Add has method

[ivan-tymoshenko]

See fastify/fastify#4986 (comment)

The has method should parse the given path in the same way as _on method does.

1. It shouldn't modify the the existing tree structure or has any other side effects.
2. Instead of throwing an error if path exists, the has method should return a boolean value here.
3. It shouldn't be a copy/paste. _on and has methods should reuse the same code or even each other.

[mcollina]

+1