feat: add i18n/a11y localisation properties to select components

[wfabr]

User-visible strings and ARIA labels in the select components were hardcoded in English, which is an issue for translation. This pull request adds overridable properties with English defaults to VscodeSelectBase, VscodeMultiSelect, and VscodeSingleSelect.

New properties

VscodeSelectBase (inherited by both select components)

• open-button-aria-label — ARIA label on the combobox toggle button (default: "Open the list of options")
• no-options-text — empty-list placeholder (default: "No options")
• create-option-prefix — prefix for the creatable entry (default: "Add", renders as Add "typed text")

VscodeMultiSelect

• selected-text — suffix in the count badge (default: "Selected", renders as "3 Selected")
• select-all-title — tooltip on the select-all button (default: "Select all")
• deselect-all-title — tooltip on the deselect-all button (default: "Deselect all")
• accept-button-text — label on the OK button (default: "OK")

Usage example

<vscode-multi-select
  open-button-aria-label="Öffne die Optionsliste"
  no-options-text="Keine Optionen"
  create-option-prefix="Hinzufügen"
  selected-text="Ausgewählt"
  select-all-title="Alle auswählen"
  deselect-all-title="Alle abwählen"
  accept-button-text="OK"
>
  ...
</vscode-multi-select>

All defaults are unchanged so there is no breaking change.

[bendera]

Thanks! I'll go thorugh it.