feat: Use margin-bottom/gap spacing in more places

[gadenbuie]

Fixes #672

• For consistency, I renamed .bslib-mb-spacer to .bslib-mb-spacing. I'm hoping this helps when trying to remember the class/variable names. The reasoning is that --bslib-spacer (the custom CSS property) is non-commital since it hasn't been applied anywhere. --bslib-mb-spacer is a similar unit. But once it's a class, we use -spacing, i.e. .bslib-gap-spacing and .bslib-mb-spacing, which I read as "the elements in this container use gap spacing or this element uses margin bottom spacing". Obviously, it's not perfect.

  • Alternative: we could use .bslib-mb or .mb-bslib instead, since there is a difference that this class directly sets the margin bottom of the current element.

• While here I added something that feels relevant: setting margin-bottom: 0 on fill items with bslib's margin bottom spacing that are activated within a fillable container.

• layout_columns() and layout_column_wrap() gain the .bslib-mb-spacing class so that they naturally have some bottom margin in a flow context. This also motivated adding the above consideration for .bslib-mb-spacing fill items.

• sidebar() gains gap and padding arguments to control the gap and padding within the sidebar itself. gap picks up from --bslib-spacer or the inline style, and I added a CSS variables --bslib-sidebar-content-padding because --bslib-sidebar-padding needs to be a single value and is used in a few places that aren't specifically a padding property. The .sidebar-content container also gains the .bslib-gap-spacing so that bottom margin is dropped in sidebars in favor of the gap spacing.

[cpsievert]

Thanks, probably worth a NEWS item?

[gadenbuie]

Thanks, probably worth a NEWS item?

I added a news item for sidebar(gap=, padding=), but we haven't really discussed the gap/margin-bottom features explicitly anywhere else. Should we consider adding a small article?