Check ascription in valdef pattern

[som-snytt]

Fixes #25595

At pattern1 and varDefOrDcl (which uses pattern2), check that the optional ascription is legal. Accept arbitrary identifiers in pattern definitions case id: T, which is already the case for the equivalent case id @ (_: T).

Correct the camelcase spelling of Elsewhere.

[som-snytt]

The breakage was due to "variable pattern" in a typed pattern versus "define X" in a valdef.

This appears simple, but currently it incorrectly accepts case X: String => instead of erroring that pattern variables must begin with lowercase. The correct syntax is case X @ (_: String) =>.

As a digression, maybe it should just officially accept case X: String that looks like val X: String. (In any other position in a pattern, it would distinguish variable ids from constant ids.) (The two forms already treat backquoted idents uniformly!)

I would have liked to add a new message with a non-trivial explanation, but the energy hump for introducing an error ID is much too high.