Refactor: put each shape into its own file (#5564)

Much easier to navigate the code

CONTRIBUTING.md

@@ -32,7 +32,7 @@ For small things, just go ahead an open a PR. For bigger things, please file an
 Browse through [`ARCHITECTURE.md`](ARCHITECTURE.md) to get a sense of how all pieces connects.
 
 You can test your code locally by running `./scripts/check.sh`.
-There are snapshots test that might need to be updated. 
+There are snapshots test that might need to be updated.
 Run the tests with `UPDATE_SNAPSHOTS=true cargo test --workspace --all-features` to update all of them.
 For more info about the tests see [egui_kittest](./crates/egui_kittest/README.md).
 
@@ -102,6 +102,7 @@ While using an immediate mode gui is simple, implementing one is a lot more tric
 * Avoid double negatives
 * Flip `if !condition {} else {}`
 * Sets of things should be lexicographically sorted (e.g. crate dependencies in `Cargo.toml`)
+* Put each type in their own file, unless they are trivial (e.g. a `struct` with no `impl`)
 * Break the above rules when it makes sense