Add a justfile

Just (https://just.systems/) works better than make for running commands. For
example, passing arguments to `make integration-test-cli` requires a `--`
between make's arguments and the cli test runner's arguments (e.g. -sandbox),
and will also result in weird errors. Just doesn't have any of these problems.

I chose different target names than we use in the Makefile; the goal is to have
better tab completion, where the most commonly used commands can be completed
after typing a single letter. That's why I use "e2e" for integration tests,
which is not a term we use anywhere else.

Keeping the Makefile around for those who are used to it, and are too lazy to
install just.

justfile

@@ -0,0 +1,57 @@
+default:
+    just --list
+
+# Build lazygit with optimizations disabled (to make debugging easier).
+build:
+    go build -gcflags='all=-N -l'
+
+install:
+    go install
+
+run: build
+    ./lazygit
+
+# Run `just debug` in one terminal tab and `just print-log` in another to view the program and its log output side by side
+debug: build
+    ./lazygit -debug
+
+print-log: build
+    ./lazygit --logs
+
+unit-test:
+    go test ./... -short
+
+# Run both unit tests and integration tests.
+test: unit-test e2e-all
+
+# Generate all our auto-generated files (test list, cheatsheets, json schema, maybe other things in the future)
+generate:
+    go generate ./...
+
+format:
+    gofumpt -l -w .
+
+lint:
+    ./scripts/golangci-lint-shim.sh run
+
+# Run integration tests with a visible UI. Most useful for running a single test; for running all tests, use `e2e-all` instead.
+e2e *args:
+    go run cmd/integration_test/main.go cli {{ args }}
+
+# Open the TUI for running integration tests.
+e2e-tui *args:
+    go run cmd/integration_test/main.go tui {{ args }}
+
+# Run all integration tests headlessly (without a visible UI).
+e2e-all:
+    go test pkg/integration/clients/*.go
+
+bump-gocui:
+    scripts/bump_gocui.sh
+
+# Record a demo
+demo *args:
+    demo/record_demo.sh {{ args }}
+
+vendor:
+    go mod vendor && go mod tidy