Adds a new embed type for code blocks that supports syntax highlighting

[skiniks]

This PR proposes a new embed type to enable code sharing in posts. This is an initial proposal to start the discussion and lay the groundwork for syntax-highlighted code blocks.

Implementation Details:

• New app.bsky.embed.code lexicon with code (max 8000 chars) and lang (max 20 chars) fields
• Follows existing embed patterns and integrates with current view system

Follow-up Considerations:

• UI/UX details (truncation, expanding, copy button)
• Additional metadata like line numbers could be added in future iterations

Interested in feedback on:

1. The lexicon structure and field limits
2. Integration approach with existing embed system