[516 ONLY] Add gap prop & replace Stack.Item margin with Stack gap

[AyIong]

About the PR

It's all said in the title. Adds props responsible for: column-gap, row-gap & gap.
This PR is fully uncompatible with Byond 515 as IE does not support gap

Why's this needed?

From what I understand, what is currently available in Stack.Item is an attempt to replace gap for IE, but since 516 is on the doorstep, I thought it would be a good idea to replace the current implementation, with a full-fledged gap.

As I wrote above, the changes are incompatible with 515, so PR will be in draft until 516 is stable and the servers finally move to it
Plus, many interfaces would have to be considered, replacing the margin cancelations in Stack.Item with margin -1 or gap 0 (Although even if you don't, nothing critical will happen)

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