Add support for row-gap and gap properties in flex layout

[kian-supportinglines]

As of #60 being fixed, we have had support for the column-gap property; I think we should also add in row-gap and gap properties. I can maybe take a look later to see how much work it would be to implement this myself, but I think it might not be so easy. Perhaps we can use the PR linked in #60 to help?

[liZe]

Hi!

Having column-gap supported for multi-column layout is only the first step. row-gap and gap work on very different environments (flex, grid…) and require a lot of changes to be handled correctly.

[liZe]

*-gap and gap are now supported for grid and multi-column layouts.

We have to rewrite the flex layout, we’ll include the support of gap then.

[benjaminuggla]

A simple way to work around this until flex support is fixed:

.item:first-child {
    margin: 0 4px 0 0;
}

.item:last-child {
    margin: 0 0 0 4px;
}

.item:first-child:last-child {
    margin: 0 0 0 0;
}

It's basically doing the same thing as gap.

[liZe]

Bug is closed and tested in the flex branch, see #2362.