Add missing types for `FastifySessionObject` function `isSaved`

[EagleLizard]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

The documentation describes the isSaved() function, and this is implemented in the plugin. The types for this are currently not included

Motivation

Enable usage in Typescript of the isSaved function without additional project configuration.

Example

A request is made that depends on new session information being in the datastore, and the datastore has constraints on a new field added by a custom SessionStore:

fastify.post('/myroute', async function(req, rep) {
  const someRes = await checkSomething(req.body.val);
  if(someRes.doUpdate) {
     if(!req.session.isSaved()) {
       await req.session.save();
     }
     await dbOpThatDependsOnSessionTable(someRes);
  }
  rep.status(200).send()
});

[EagleLizard]

I am more than happy to open a PR for this, but I lack access to create a branch.