Split off from T423858. See design.
Acceptance criteria
- add pulsing blue dot when share button in toolbelt is first seen. Refer to prior work in VisualEditor
- apply accessible label: readerexperiments-sharehighlight-blue-dot-label
- i18n message registered in this patch: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/ReaderExperiments/+/1282394
- dismiss blue dot after the user has interacted with the share button once - record that in LocalStorage/cookie/... (have a look elsewhere how similar things have been done)




