Add gap style

[mhsmith]

• Fixes Equivalent of CSS gap (padding between children) #1943

This PR is stacked on top of #3033, which should be merged first. To view only the relevant changes on the "Files changed" page, open the commits menu at the top left and use shift-click to select a range of commits.

I haven't updated all the examples that could benefit from this, but I've done one each with a row and a column layout.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[HalfWhitt]

@mhsmith Some more commits over on #3033 now

[mhsmith]

gap would interact with potential future column_gap and row_gap properties in the same way as margin interacts with margin_left, etc.:

• You could set gap to an integer to set both gaps to the same value. This is the only option at the moment, because we only support 1-dimensional containers.
• Or set gap to a 2-tuple to set them to different values.
• Or set column-gap and row-gap individually.

[HalfWhitt]

gap would interact with potential future column_gap and row_gap properties in the same way as margin interacts with margin_left, etc.:

Thank you for pointing this out... directional_property's current implementation in Travertino would eventually need some generalizing to handle this.

[freakboy3742]

👍 Looks exactly like what I'd expect.