rustfmt, clippy, gh actions, justfile, Default app (#4)

* `Default` app
* add zed settings
* add justfile
* flake: update devShell, ignore tests, add `just`
* update README
* gh actions: lint, format, tests. build

justfile

@@ -0,0 +1,32 @@
+# The `--fmt` command is currently unstable.
+
+set unstable := true
+
+default: run
+
+alias b := build
+alias f := format
+alias l := lint
+alias t := test
+alias r := run
+
+# build app
+build:
+    cargo build
+
+# run tests
+test:
+    cargo test
+
+# format files
+format:
+    just --fmt
+    cargo fmt --check
+
+# lint
+lint:
+    cargo clippy --no-deps
+
+# run app
+run:
+    cargo run