list_view: add optional multi-select mode

list_view supports a single selected index (nk_console_list_view_set_selected, #254). There is no way to select multiple items, which limits use cases like bulk file ops or settings that apply to a subset of a list.

Suggested Implementation

• Add nk_console_list_view_set_multi_select(widget, nk_bool enabled).
• When enabled, maintain a nk_bool* selected mask alongside the existing selected int.
• Pressing A toggles the focused item in the mask; existing single-select behavior unchanged.
• Add nk_console_list_view_is_selected(widget, int index) accessor.

QA

Multi-select enabled: pressing A on items 0, 2, 4 marks all three. Single-select still returns one index via the existing getter.

Start from nk_console_list_view_set_selected and the list_view input handling, then trace how the existing selected index is stored and changed when A is pressed. Add the multi-select toggle and nk_console_list_view_is_selected accessor described in the issue. Done means items 0, 2, and 4 remain selected after toggling, while single-select still exposes one index through the existing getter.