Work-in-progress Jest support [DO NOT MERGE]

[RichardGale]

Commit for future reference, jest plus svelte plus typescript does not work yet as jest requires $app namespaced modules from svelte that do not exist in the jest environment.

To run:

npm install npm jest

but will fails at: Cannot find module '$app/stores' from 'src/lib/header/Header.svelte'

Require stack:
  src/lib/header/Header.svelte
  src/__tests__/Modal.spec.ts

  1 | <script lang="ts">
> 2 |   import { page } from "$app/stores";
    | ^

Closes #

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• [ ] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [ ] I have updated the documentation as required
• [ ] All the tests have passed (TDB)

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