Pre-release of Typescript & Composition API re-write

Shout out to @akki-jat for doing the work needed to get this done. Check out the README on the branch to see more details: https://github.com/shentao/vue-multiselect/tree/v4

Features

• Rewrite source from JS Options-API mixins to TypeScript composables (useMultiselect, usePointer) with a <script setup> SFC; component behaviour, props, events, slots and styles unchanged.

Breaking Changes

• Removed the multiselectMixin/pointerMixin exports (use the useMultiselect/usePointer composables);
• Dist filenames are now vue-multiselect.esm.js/.cjs/.umd.js/.css;
• Requires Vue ^3.5 and Node >=20.19.

Internal tooling changes

• Replace Vue CLI + Rollup with Vite library mode (esm/cjs/umd + generated .d.ts); migrate Jest -> Vitest and ESLint -> flat config; bump all deps.
• Regenerate docs (Vite 8, shiki 4) and dist artifacts.

Linked PRs

• feat!: TypeScript + Composition API rewrite (v4.0.0-alpha.0) by @akki-jat in #1943

Full Changelog: v3.5.0...v4.0.0-alpha.0