feat: add slot to gain control over tabs switching buttons

[OlkaB]

Contributes to #1522

## What did you do?
Added slot to customise tab switching buttons' content

How have you tested it?

added test for slot in CvTabs.spec.js

Were docs updated if needed?

• Yes

sample:

[davidnixon]

I like this solution but I think using the label is a problem. The Label would often be localized so the "House" label might become "Taigh" or "Maison" or "Casa". Could you use the id instead?

[OlkaB]

fair point - change applied

[davidnixon]

sooooooo close:
"To customise tab look, in CvTabs component use scoped slot named with tab's label. Example: current tab has label "House", so the code for customising tab looks like:"

need to switch text to id 😄

[OlkaB]

oh gosh - truly. Done, thanks @davidnixon

[davidnixon]

Thanks!