Fix #5986: Add autolift implicit conversion

[biboudis]

Add an autolift implicit conversion and update tests.

Eliminating .toExpr doesn't give always good intuition about what kind of values exist inside a splice.
For example with autolift the following code has x in both cases. One of them is implicitly lifted and the other is not 😛.

def dyn[T: Liftable](pv: PV[T]): Expr[T] = pv match {
  case Sta(x) => x 
  case Dyn(x) => x
}

WDYT?

[odersky]

I think that's fine since normally we do not care whether something is lifted or not.
I'd even go further and include the autolift conversion in quoted. If one does not want it, one can still write

import scala.quoted.{autolift => _, _}

[nicolasstucki]

We need to add this to principled-meta-programming.md. Probably a new paragraph at the end of the The Liftable type-class section.

[nicolasstucki]

Could you also change the commit message to Fix #5986: .... Github does not detect it on the PR title.

[biboudis]

Rebased, updated the doc and left the automatic way out for now.

Addressed the documentation