Add new default macro to declare read-write variables

[PGimenez]

trafficstars

@essenciary @hhaensel

Right now we have @in and @out to declare read-write and read-only variables. Many times I've declared an @out variable by mistake and then wondered why the UI component it was bound to wasn't working. I assume this could happen to other people as well.

I'd like to have a different macro that would declare read-write variables and is named something other than @in, to avoid having to think whether the variable is read-only/read-write. Something like @io, @bind, @state.

And now that I think about it, given its name, @inshould only take values from the browser to the backend and not the other way around. But this would be a breaking change for people's apps so let's not do that.