Turning the "importsNotUsedAsValues" TS config value to "error".
This will require us to use `import type` instead of `import` when we are importing a value that is only used as a type (and therefore that is dropped by the Typescript compiler).
Why this change?
This is a requirement to be able to use Svelte in the future. See https://github.com/sveltejs/svelte-preprocess/issues/206#issuecomment-663193798
If users are willing to use Typescript to develop scripts for maps, they will need typings for the `WA` object.
This commit creates a new package (@workadventure/iframe-api-typings) that contains only the `iframe_api.d.ts` file.
The file is generated from the build of the front and isolated in this package.
This is necessary because the iframe_api.js file is supposed to always be loaded from WorkAdventure directly (and there is no @workadventure/iframe-api package and there will never be one)
