Describe the bug

As recently as version 0.25.6, you could use a foot slot on BTable for custom footer cells, as shown here:
https://stackblitz.com/edit/github-jwsdlr-gb3spw?file=src%2Fcomponents%2FComp.vue

In version 0.25.12, that no longer works as shown here:
https://stackblitz.com/edit/github-jwsdlr-4lmgya?file=src%2Fcomponents%2FComp.vue

It looks like there were some recent BTable changes that may have broken it, like this one:
e51461e

There was also a commit today for a new custom-foot slot that hasn't been published to npm yet. I haven't tested that, but regardless the slot name is not the same:
264fc9e

cc @VividLemon @tbl0605

Reproduction

https://stackblitz.com/edit/github-jwsdlr-4lmgya?file=src%2Fcomponents%2FComp.vue

Used Package Manager

pnpm

Validations

• Have tested with the latest version. This is still alpha version and sometime things change rapidly.
• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A, please open a GitHub Discussion instead.
• The provided reproduction is a minimal reproducible of the bug.