Fix plugin type in config/PrimeVue.d.ts
=======================================
vue 3 does not export a `PluginFunction` type. The `install` function should be of type `PluginInstallFunction`, but even that isn't exported. Instead we declare the type of the default export as `Plugin`.
See
https://github.com/vuejs/vue-next/blob/f0cf14bcc56c387372932e7d730f838ece17fe5f/packages/runtime-core/src/apiCreateApp.ts#L90-L96.
Fixes #1164.
primevue/config: TypeScript error due to unexported type
========================================================
When importing `primevue/config` in TypeScript, the following error occurs:
```
Module '"vue"' has no exported member 'PluginFunction'.
```
This is because vue 3 does not export a `PluginFunction` type. Only `Plugin` is exported.
https://github.com/vuejs/vue-next/blob/f0cf14bcc56c387372932e7d730f838ece17fe5f/packages/runtime-core/src/apiCreateApp.ts#L90-L96