Documented implicit events not picked up by vue-docgen

The events emitted by the GlFormSelect component are documented in !2556 (merged). Unfortunately, vue-docgen ignores @event tags when not immediately above an explicit $emit call, and because GlFormSelect is a transparent wrapper around BFormSelect, it has no explicit $emit calls.

This will be a fairly common problem for GitLab UI's components, so we should come up with a fix or workaround as part of the CSF migration.