NIP-91 : AND Operator in Filters

Enable AND within a single tag filter by using an & modifier in filters for indexable tags.

filters: {
  "kinds": [1],
  "&t": ["meme", "cat"],
  "#t": ["meme", "cat", "black", "white"]
}
// returns kind `1` events with `t` tags that have both "meme" and "cat" that have the tag "black" or "white" by NIP-91 relays, whereas relays without NIP-91 return entire result for local intersection.

Rules

• AND MUST take precedence over OR by relays
• Any tag value used in AND SHOULD be ignored in OR by relays supporting NIP-91
• Tag values used in AND by libraries and clients MUST include standard OR tags [#] for compatibility with relays that do not support NIP-91.
• libraries and clients SHOULD run & post-filters after recieving results from relays; already standard practice.

Copied from the pending PR