[Core] Add new using function

[drkameleon]

Right now, we have with which basically creates a new block by encapsulating a given variable/value (very useful in a "closure" context).

What using would do is something similar-ish, but:

• safer
• totally scoped
• more specific
• centered around this

Also, using as a name is used in very similar contexts in other languages... So...

In short: the idea is to... introduce a forced this (and restore the previous one, if there), so that we can use field \shortcuts.
As similar idea is Nim's own with: https://nim-lang.org/docs/with.html#with.m%2Ctyped%2Cvarargs%5Buntyped%5D

And, yes, it's more than just a simple coincidence... lol (👀 #1731)

Example:

; before
window\size: [300, 400]
window\center
window\title: "New window"

; after
using window [
    \size: [300, 400]
    \center
    \title: "New window"
]

[RickBarretto]

What a great addition!
This would improve the readability and workflow. ❤️