Single line case lambda gets a region

[som-snytt]

Fixes #24496

[som-snytt]

In the spirit of, "People will try it":

  val one = list
        .collect: case x =>
          (x, x + 1)
        .toMap

Maybe it's enough to detect arrow at EOL. (Edit: "People will already have used it.")

Edit: actually, the SIP isn't very plain on this point, but the PR includes a test that case expr is multiline.

This is OK:

  val two = list
        .collect: x => (x, x + 1)
          .toMap

There is no misalignment warning.

[som-snytt]

Closing to verify which behavior is wanted.