quadlet: add support for multiple quadlets in a single file

Enable installing multiple quadlets from one file using '---' delimiters.
Each section requires '# FileName=<name>' comment for custom naming.
Single quadlet files remain unchanged for backward compatibility.

Assited by: claude-4-sonnet

Signed-off-by: flouthoc <flouthoc.git@gmail.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f