Add instructions for type-only imports

[jmcooper]

Description of Problem

Because Vue uses isolatedModules, it is necessary to use type-only import syntax when importing types. Discovering the resolution for this edge case is difficult when first beginning to work with TypeScript and Vue/Vite.

This is easy to replicate by creating an app with create-vue and selectihg the typescript option. Then, export an interface from a .ts file and import it and try to use it as a type in a component. When you run npm run type-check you will see the error. Here is a sample repo that duplicates the problem with a new create-vue app: https://github.com/jmcooper/create-vue-ts-repro

Reproducing the Problem

See instructions above to repro from scratch. Or:

1. Clone https://github.com/jmcooper/create-vue-ts-repro
2. npm install
3. Run npm run type-check

Proposed Solution

Update the documentation as proposed to make it clear this is an expected behavior and how to resolve it.

Additional Information

Sample repo to replicate the problem: https://github.com/jmcooper/create-vue-ts-repro

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