Adds native intersection type handling to psalm, removing the previous `UnexpectedValueException`.

Where an intersection is found in the parse tree, the types are resolved using the existing `Type::intersectUnionTypes` function, which I assume is being used when they're encountered in the existing docblock annotations.

I've added a handful of tests to cover this, but they're certainly not exhaustive. Are there any specific edge cases I should target?

This change feels way too simple... so apologies if I've missed something fundamental..!