Implement support for optionally specifying the 'var' keyword in 'for' loops

[gunnarbeutner]

This adds support for optionally using the var keyword in for loops:

for (var number in [ 4, 8, 15, 16, 23, 42 ]) {
    log(number)
}

and

for (var key => var value in globals) {
    log(key)
    log(typeof(value))
}

Adding the var keyword has no semantic effect other than unifying how we usually do variable declarations.

[Crunsher]

lgtm 👯‍♀️

[dnsmichi]

From a first look, this makes it complicated to follow and understand e.g. apply for inside the monitoring basics chapter. Let's talk about this next week in the office :)

[dnsmichi]

It can be added, nevertheless we shouldn't change our entire documentation for it. If that's the requirement, I'd drop this feature as this makes it complicated for our users (who are not programmers).

[gunnarbeutner]

I guess I might reword this to drop all the docs changes (apart from the bit where we mention that users can optionally add var in their loop var decls).

[gunnarbeutner]

I've updated the PR to remove most of the controversial doc and example config changes. 🙂

[gunnarbeutner]

Any objections to setting the target to 2.10?

[dnsmichi]

Looks good, thanks :) Review/merge happens after 2.9.