Warn negative literal parsing under -Wsyntax

[som-snytt]

Introduce -Wsyntax to warn about syntax warts.

Warns about - 42 (the misconception that space is significant);
and -42.abs (the misconception that it parses the same as -x.abs, i.e. -(42.abs)).

Tweak prefixExpr to exclude subsequent expression on the next line.

Tweak simplePattern to exclude backquoted hyphen as unary minus.

Fixes #7910
Fixes #24162

[som-snytt]

Resuscitates #12459

Addresses the question at https://contributors.scala-lang.org/t/functional-syntax/5062/1

Could use code actions.