feat: migrate to Vue 3 and Vue Test Utils v2

[GerardoIbarra]

This PR migrates the Vue-select library from Vue 2 to Vue 3, including updates to the core component, build configuration, and the entire test suite. The component is now fully compatible with the Vue 3 reactivity system and modern tooling.

Key Changes

Core Component ( Select.vue )

🟢 Renamed value prop to modelValue for Vue 3 v-model compatibility.
🟢 Updated event emissions:
- input→ update:modelValue.
🟢 Replaced $scopedSlots with $slots (unified slots API).
🟢 Added explicit emits declaration for all internal events.
🟢 Updated dynamic ref handling for v-for loops (deselect buttons).

Test Infrastructure

🟢 Migrated from vue-jest to @vue/vue3-jest and updated babel-jest.
🟢 Updated
tests/helpers.js
to use createApp and h instead of the global Vue instance.
🟢 Refactored all 22 test suites (174 tests) to use Vue 3 compatible syntax and selectors.
🟢 Migrated slot tests to the new function-based render API.

Build & Tooling

🟢 Replaced .babelrc with babel.config.js for better Jest compatibility.
🟢 Updated Webpack aliases to use the Vue 3 ESM bundler.
🟢 Updated package.json dependencies:
- vue(^3.5),
- @vue/test-utils (^2.4),
- and added @vue/compiler-sfc.

Verification Results

✅ Unit Tests: 100% test suites passing (172 passed, 2 skipped due to JSDOM environment limitations).
✅ Dev Server: Verified working via npm run serve.
✅ Build: Production build completes successfully.

Breaking Changes

• This release is not backwards compatible with Vue 2.
• Users must update their v-model usage from value to modelValue (or just use standard v-model).