Enable Artemis to hook into the React render timeline for more reliable tests

Checklist

[X] I documented the TypeScript code using JSDoc style.

Motivation and Context

To fix flaky client tests related to Apollon in #6955, I need to be able to wait for the next render cycle in React. To my knowledge, this is not possible without some "help" from the React code itself (or with a React testing library, which we don't use). Therefore, we need this PR.

Description

Added a new promise to the ApollonEditor called nextRender, which resolves when the next scheduled React render of the editor component happens. It also ensures that ApollonEditor.store will be properly initialized before it resolves.

Steps for Testing

Test https://github.com/ls1intum/Artemis/pull/6955 using @palsch/apollon:2.14.12, where this PR is available as an NPM package.
Run npm start locally to check that the standalone editor still works.

Test Coverage

File	Line
apollon-editor.ts	82%
application.tsx	100%

ls1intum / Apollon