Macro syntax consistency with AbstractPlotting

[piever]

The "helper macros" here use the & syntax to denote observables that trigger an update, e.g. @map &x + &y (see docs). At the time, this was somewhat inspired by the ampersand operator in C, even though tbh I'm not able to give a consistent argument in favor of it at the moment.

OTOH, in AbstractPlotting, which uses a very similar macro, the observables that trigger an update are denoted by $, e.g. @lift $x + $y (see docs).

From what I understand, Observables is going towards a 1.0 release, so I thought it was relevant to figure out whether we want a unified syntax and, if it is Observables that should change, make the change before tagging 1.0.

[timholy]

I haven't followed this area at all myself, but that sounds good to me. My current thinking is that currently we have one deprecation (notify!). Perhaps we should call the next release 0.4.0, and then when we delete the deprecation we release 1.0. That gives us a chance to play with the new version for a bit and decide whether there are any other important API changes.