feat: add focus and keyboard resize support to split-layout

[web-padawan]

Description

Fixes #951

• Added role="slider" and tabindex="0" to the splitter element making it focusable
• Added aria-valuemin, aria-valuemax and aria-valuenow announced by screen readers
• Added FocusMixin with focus() override, focused and focus-ring attributes logic
• Added CSS to apply outline on the splitter if focus-ring is set (base styles and Lumo)
• Added I18nMixin and i18n property { separator: 'Resize separator' } by default
• Added KeyboardMixin and logic for Arrow keys (+ RTL), PageDown / PageUp, Home / End
  • Arrow keys use a hardcoded default step of 16px, no custom step for now
  • PageDown / PageUp keys use 10% as a step (evaluated using available size)
• Keydown events are debounced and reuse existing splitter-dragend event to notify change
• Added logic to focus the splitter on pointer up event to allow pressing keys after clicking it

Type of change

• Feature

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud