Fix CASE expression spans

[eliaperantoni]

Previously, a Expr::Case implementation of Spanned would ignore the CASE .. END keywords. ie.

CASE a WHEN b THEN c ELSE d END
--   ^^^^^^^^^^^^^^^^^^^^^^
--   Only this would be highlighted

Because the only stored locations would be those of the Ident inside the CASE expression.

This PR attaches two AttachedToken to the AST node, one for CASE and one for END, so that Spanned::span now returns the whole range. This was done to follow the footsteps of the SELECT implementation, which also stores an AttachedToken for the SELECT keyword. Also added a test for this.

Closes #1874.

[iffyio]

Thanks @eliaperantoni! The changes look reasonable to me, left a minor comment

[iffyio]

LGTM! Thanks again @eliaperantoni!
cc @alamb