feat: get rid of context and made useModal can be execute everywhere

[hunterliu1003]

Made useModal truly allowed to execute everywhere

1. Vfm should provide a way to create a modal instance outside of setup script, like a singleton of a dynamic modal, for example:

   // modalHelpers.ts
   export const modalInstance = useModal({
     ...
   })

   and import the modalInstance from else where:

   <script lang="ts" setup>
   import { modalInstance } from './modalHelpers' 
   
   modalInstance.open()
   </script>

2. Vfm should provide a way to create a modal that opened by default, for example:

   // modalHelpers.ts
   export const modalInstance = useModal({
     defaultModelValue: true,
     ...
   })

   and also open the modal right after create the modal instance, for example:

   // modalHelpers.ts
   export const modalInstance = useModal({
     ...
   })
   modalInstance.open()

3. The PR also delete the context: vfm and replace it with the getActiveVfm() pattern that inspired from pinia

[hunterliu1003]

Known issue: the PR made the SSR of modal broken

[hunterliu1003]

Known issue: the PR made the SSR of modal broken

fixed by this commit https://github.com/vue-final/vue-final-modal/pull/355/commits/c134d8ae55c7285de7e300ce4450c6ead317fc16