This commit introduces a new command `borg collect discord import` to
import and archive Discord server exports.
The command processes a JSON export from tools like DiscordChatExporter,
converts the messages into Markdown files organized by channel,
downloads all attachments to create a self-contained archive, and
generates a searchable `INDEX.json` file.
Key features:
- Parses DiscordChatExporter JSON format.
- Converts chat logs to Markdown, preserving metadata.
- Downloads and localizes all message attachments.
- Creates a searchable `INDEX.json`.
- Sanitizes server and channel names to prevent path traversal.
Co-authored-by: Snider <631881+Snider@users.noreply.github.com>
