Fix a bug where a GPUI macro still used `ModelContext`
Rename `AsyncAppContext` -> `AsyncApp`
Rename update_model, read_model, insert_model, and reserve_model to
update_entity, read_entity, insert_entity, and reserve_entity
Release Notes:
- N/A
`cargo run` on Zed project leads to rust-analyzer evantually emitting
`[ERROR project_model::workspace] cyclic deps:
gpui_macros(Idx::<CrateData>(269)) -> gpui(Idx::<CrateData>(264)),
alternative path: gpui(Idx::<CrateData>(264)) ->
gpui_macros(Idx::<CrateData>(269))`
error after loading the project.
The PR fixes this by moving away the test to the "root" project.