Describe the PR

Fix #1822 (Empty string as entry in options array of BFormCheckboxGroup returned as true in v-model) by inlining the string | boolean part of the CheckboxValue type in the defineProps of BFormCheckboxGroup.vue.

According the the vue docs, the way to disable the boolean casting rule in this case is to make sure string comes before boolean in the type union. Changing the type definition in CheckBoxTypes.ts wasn't sufficient because the compiler-sfc doesn't crawl external files, so prepended the string | boolean part of the union inline in the place that compiler-sfc uses. This is somewhat ugly, but I believe it is better than the alternatives that I considered:

• Redefine CheckboxValue in BFormCheckboxGroup.vue
• Move the definitions in CheckboxTypes.ts to BFormCheckboxGroup.vue and then make index.ts in types refer to those definitions

Small replication

See #1822

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix 🐛 - fix(...)
• Feature - feat(...)
• ARIA accessibility - fix(...)
• Documentation update - docs(...)
• Other (please describe)

The PR fulfills these requirements:

• Pull request title and all commits follow the Conventional Commits convention or has an override in this pull request body This is very important, as the CHANGELOG is generated from these messages, and determines the next version type. Pull requests that do not follow conventional commits or do not have an override will be denied