Generate mirrors for named tuples

[jchyb]

Fixes #22382
For summon[Mirror.Of[(foo: Int, bla: String)]] we generate:

new scala.runtime.TupleMirror(2).$asInstanceOf[
    scala.deriving.Mirror.Product{
      type MirroredMonoType = (foo : Int, bla : String);
        type MirroredType = (foo : Int, bla : String);
        type MirroredLabel = ("NamedTuple" : String);
        type MirroredElemTypes = (Int, String);
        type MirroredElemLabels = (("foo" : String),
          ("bla" : String))
    }
]

We reuse scala.runtime.TupleMirror, because it pretty much does everything we want it to, and fromProduct (with supplied Product types) call on that mirror still works there.
Since NamedTuple is not technically a Product type, I imagine users might be a little confused why they can't put a named tuple into a fromProduct argument, but this is easily worked around with .toTuple

[bishabosha]

Suggested change

	case tp @ AppliedType(tref: TypeRef, List(_, typeTuple)) if tref == defn.NamedTupleTypeRef =>
	case tp @ AppliedType(tref: TypeRef, List(_, _)) if tref == defn.NamedTupleTypeRef =>

[jchyb]

I ended up changing it into defn.NamedTupleDirect(_, _) (same AppliedType internally), for readability (I don't think that unapply was there when I started making this PR)

[bishabosha]

what changes with derived=true instead of false?

[jchyb]

derived=true allows to convert union and intersection types of NamedTuples into NamedTuple AppliedType types. We already know it's a NamedTupleDirect from the unapply above, so we don't need to do that (and setting true would not change anything here).

[bishabosha]

Suggested change

	makeProductMirror(typeElems, elemLabels, "NamedTuple".toTypeName, mirrorRef)
	makeProductMirror(typeElems, elemLabels, tpnme.NamedTuple, mirrorRef)

if this doesnt exist it should

[bishabosha]

Suggested change

val cls = if arity <= Definitions.MaxTupleArity then defn.TupleType(arity).nn.classSymbol else defn.PairClass

unused? (is there a side-effect?)

[bishabosha]

(also unused)