fix(editor): preserve leading newlines in plain text editor

[omnizs38]

📝 Summary

• Resolves: N/A

This PR fixes an issue where the plain text editor stripped leading newlines and trailing whitespace on empty first lines of plain text documents.

When setting or updating the content of the plain text editor, the raw content is escaped and wrapped inside HTML <pre>...</pre> tags. By standard HTML rules, DOMParser strips a single leading newline character inside <pre> tags. Prepending a newline (\n) to <pre> content ensures that DOMParser strips only our prepended newline, leaving the document's original leading newlines and whitespace intact.

This change also re-enables the previously skipped CommonMark plain text tests 97 and 117, which now pass successfully.

🖼️ Screenshots

🏚️ Before	🏡 After
N/A (No visual changes)	N/A (No visual changes)

🚧 TODO

• None

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI tools
• The AI-generated content was reviewed, comprehended and tested by a human