Add a Makefile with build, test and run targets #84

Add a Makefile at the repo root for this Cargo project (binary name hello). Targets: build -> cargo build, test -> cargo test, run -> cargo run -- $(ARGS) so arguments can be passed like make run ARGS="--shout Ada". Declare them .PHONY, make build the default goal, and use tab-indented recipes. Do not edit other files.

Definition of done

• make build passes.
• make test passes.
• make run ARGS=Ada passes.