feat: make hardcoded UI strings and ARIA labels configurable for i18n/a11y

[Copilot]

All user-visible strings and ARIA labels in the select components were hardcoded in English, making the library unusable in non-English locales or custom accessibility contexts. This 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.

[wfabr]

Approved merge