feat: add Select All / Deselect All support for multi-select

[govindarajug]

Summary

Adds an opt-in enableSelectAll prop that prepends a "Select All" / "Deselect All" option at the top of the menu when isMulti is true. This is one of the most commonly requested features for react-select based libraries.

Usage

// Basic — default labels
<WindowedSelect isMulti enableSelectAll options={options} />

// Custom labels
<WindowedSelect
  isMulti
  enableSelectAll={{
    selectAllLabel: 'Check All',
    deselectAllLabel: 'Uncheck All',
  }}
  options={options}
/>

Behaviors

• Label toggles between "Select All" and "Deselect All" based on whether all visible options are selected
• Respects search filtering — only selects/deselects options matching the current search input (e.g. type "foo", click Select All → only options matching "foo" are selected)
• Works with grouped options — flattens groups for comparison, inserts the toggle at the top
• Custom labels — pass { selectAllLabel, deselectAllLabel } to override defaults
• Composes with custom Option components — user-provided components.Option is preserved for non-select-all rows
• No effect on single-select — feature is gated on isMulti
• Works with custom getOptionValue — respects the provided value accessor
• Deselect All preserves non-filtered selections — if user searches "A" and clicks Deselect All, only "A"-matching selections are removed

Changes

• WindowedSelect.tsx — new enableSelectAll prop, option injection, onChange interception, Option component wrapping
• util.ts — new flattenOptions utility for extracting options from grouped structures

Related issues

This addresses one of the most requested features in the react-select ecosystem:

• Select All Option JedWatson/react-select#3543 (23 comments) — "Select All Option"
• How can i create options with checkbox and toggle selected if their checkbox is checked? JedWatson/react-select#2032 (22 comments) — "Create options with checkbox and toggle"
• "Select all" for multiselect JedWatson/react-select#747, #3668, #3860, #2263 — duplicate requests

Test plan

• Multi-select with 200+ options: "Select All" appears at top, selects all on click
• After selecting all, label changes to "Deselect All", click deselects all
• Type a search term, click "Select All" — only filtered options are selected
• Type a search term, click "Deselect All" — only filtered options are deselected, others remain
• Grouped options: Select All appears above first group, selects across groups
• Single-select with enableSelectAll: no Select All option shown
• Custom getOptionValue: Select All correctly identifies selected state
• Custom components.Option: preserved for regular options, select-all row uses wrapper