Make GlCollapsibleListbox and GlDisclosureDropdown render correctly inside GlButtonGroup

We have several instances while migrating to new dropdowns, where developers needed to workaround the missing support for the split toggle.

We don't want to implement the split prop in these components as it adds complexity to already complex components. Instead, we'll rely on composition with GlButtonGroup.

GlDisclosureDropdown already seems to implement this. We just need to do the same for GlCollapsibleListbox.

Requirements

• The implementation for GlDisclosureDropdown should be reviewed as part of this work.
• It should be sufficient to put a button and one of these dropdowns inside a GlButtonGroup and border radius/gaps should be correct without needing to apply additional classes.
• Non-default adjacent dropdown triggers should have a 1px gap: gitlab!124916 (comment 1460557497).
• Existing uses of GlButtonGroup with these dropdowns should be reviewed and cleaned up after this work has been done. Might require an epic as a follow-up.

Instances

1. gitlab!122583 (comment 1422333018)
2. gitlab!124916 (comment 1461672914)
3. gitlab!133402 (merged)
4. gitlab!135283 (comment 1621534111)

See also

• Consider how to implement rounded corners with ... (#2250 - closed)