feature 'Vec2' struct

[faretek1]

it would be useful to have a struct for managing 2d (and 3d) vectors as a standard. due to the nature of scratch being based on a co-ordinate grid, it is only natural for such a struct to exist, and it already exists in various projects in slightly varying forms.

It would have 2 attributes:

• x
• y

[faretek1]

this repo: https://github.com/inflated-goboscript/vec2 has some vec2 stuff based on 3blue1brown's series