experiment: add tabs base styles and visual tests

[jouni]

Built-In Variants

Name	Description
hide-scroll-buttons	Always hides the scroll buttons, also on devices with pointer: fine.
show-scroll-buttons	Always shows the scroll buttons, also on devices with pointer: coarse (touch devices).

Supported Custom Properties

Tab

Property	Fallback value
--vaadin-tab-gap	Gap between tab icon and label.
--vaadin-tab-padding
--vaadin-tab-font-size
--vaadin-tab-font-weight
--vaadin-tab-line-height
--vaadin-tab-color
--vaadin-tab-background
--vaadin-tab-border-radius

Tabs container

Property	Fallback value
--vaadin-tabs-gap	Gap between tabs.

[jouni]

Not sure exactly what to do with the unit tests (yarn test --group=tabs --theme=base), as they seem to rely on the Lumo styling (some 20px magic number) and seem to completely fail on some getBoundingClientRect() call.

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot

See analysis details on SonarQube Cloud