Fix Task to Markdown Conversion

[Clashsoft]

Bugfixes

• Switching between task list and markdown editor now preserves all structure and information correctly. Subtasks become Sub-Subtasks when switching between Markdown and List #441

Closes #441

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frontend/src/app/assignment/modules/edit-assignment/task-markdown.service.ts:72

• The extra variable should be included in the rendered markdown for both headlines and non-headlines to ensure consistency.

return `${headlinePrefix} ${description} (${points}P)<!--${extra}-->
${childrenMd}`;