R1:20aad35e6036

Move Ferret engine "title:..." field definitions to the engine itself

Summary: Ref T12819. Move these out of the core engine into the Ferret engine. In the future different applications can define different functions, like "summary:..." or whatever. This may get more formalization when I possibly do "author:" and such some time down the road.

Test Plan: Searched for "title:...". Searched for "dog:...", got a useful error.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision…