Filter/Where as a standalone concept

Nobody has claimed this yet.

Refactor the current implementation of LoopBack's Filter to allow other packages to leverage the filtering implementation and/or support LoopBack's Filter syntax.

1. The syntax of Filter/Where objects should be well defined and properly documented, to allow 3rd-party implementations to understand all possible values.

2. There should be TypeScript typings describing Filter, Where, etc., these typings must be accessible independently on `@loopback/repository` (e.g. via a new package).

3. The implementation used by our memory connector should be refactored/extracted into a package that can be used independently from juggler (something like https://www.npmjs.com/package/loopback-filters). It's important to update the connector to use that new package, to ensure there is only single reference implementation.

Nice to have:

- JSON Schema describing Filter syntax
- Some sort of a test suite to allow 3rd-party implementations to verify conformance

Begin by locating LoopBack's existing Filter/Where implementation and the memory connector in juggler, then review how @loopback/repository exposes its types. Done means the syntax is documented, independent TypeScript typings and a reusable filtering package exist, the memory connector uses the shared implementation, and the proposed JSON Schema or conformance tests are addressed.

Written by the indexing model from the issue text.