Add "contentIsEmpty" query field for deleted documents

[cinnamon-bun]

What's the problem you want solved?

"Deleted documents" are docs with content: "".

We need to preserve them behind the scenes, as tombstones.

Sometimes we want them (when syncing), and sometimes not (when showing things in a UI).

Is there a solution you'd like to recommend?

• [x] Add a query field which controls if they are included in results or not.
• [x] Decide what the default value is
• [x] Implement in memory storage
• [ ] Implement in sqlite storage (partially working, blocked on #44 )
• [ ] Figure out how it interacts with document history (blocked on #44 What should "includeHistory" actually do?)

[cinnamon-bun]

Added in https://github.com/earthstar-project/earthstar/commit/34e9c758223fdf31250738f676eae73597132b90

    // If true, only match documents with content === "" (e.g. deleted documents)
    // If false, only match documents with content.length >= 1
    // If omitted, match all documents.
    contentIsEmpty?: boolean,

Tests pass with memory storage, but not all tests pass with sqlite storage. We're stuck waiting to figure out #44 to define what the right behavior should be.