Deprecate `new` keyword WRT `new SomeClass`

Unlike `const`, there are no places (that I can think of) where this would still be needed to instantiate a type.

Start by reading issue #2401 and the Dart language design context; the payload names no files, tests, or entry points. Determine the intended scope and compatibility impact of deprecating `new` before identifying the relevant specification work. Done would require a decided language-design outcome, but the issue does not define one.

Written by the indexing model from the issue text.