Add Notion page search support

[spacephoton]

• Simple Notion page search.
• Uses beta search API to find pages and database titles: https://developers.notion.com/reference/post-search.

Caveats:

• Notion API sometimes returns duplicate results when query matches several properties (such as url + title).
• Only included page results because not clear how to create urls from the database results, and users are more likely to search for pages within the database anyway.
• Did not include content preview, which could be fetched with the block children api. Page contents may not contain the search query since search API is searches through page titles and properties.

[artnc]

Thanks, this is great!