Deprecate support for the comma operator outside of for() conditions and increments

See issue 2659 [1]. The comma operator is a source of hard-to-detect bugs, is mostly unnecessary, and blocks the path to a nice template literal syntax. The only place where it makes sense to keep it is in the condition and increment parts of `for` statements.

Is it better to keep the functionality and deprecate it first?

(This PR depends on https://github.com/D-Programming-Language/phobos/pull/2041 for Phobos to compile again.)

[1] https://d.puremagic.com/issues/show_bug.cgi?id=2659