Vue3 upgrade: launchdarkly-js-client-sdk

Investigate/identify:
- whether we can use this library in Vue3
- what is missing to be able to use it (or when it will be available)
- alternate packages (or whether we can change our code as needed)
- what if we stop using this package altogether?

Start by investigating whether launchdarkly-js-client-sdk supports Vue 3 and identify any missing capabilities or compatibility constraints. Compare alternate packages and the implications of changing or removing this package. Done means documenting the compatibility findings, viable alternatives, and a recommendation.

Written by the indexing model from the issue text.