[v4] Consider renaming for to each

[gilbert]

When users see <for(var x in xs)>, they think it's native JavaScript and try to do fancy ES6 stuff like destructuring.

Here's an actual attempt:

<for(const [airport, { area, name, pos: { lat, lng } }] of Object.entries(geo.airports))>

I have two suggestions (not mutually-exclusive):

1. Rename for to each to avoid confusion
2. Give for native JS code support

This would be the last nail in the "uncanny JavaScript-like tags" coffin.

[mauricionr]

Or could be more vue like

for((key, value) in data)//object

And

for(item in data)//array

Thanks :)

[DylanPiercey]

We've switched up the for tag syntax keeping this in mind in #1238