Add general purpose GNUmakefile

[ingydotnet]

This GNUmakefile auto-installs (unless go is installed already) go locally in .cache/local/go-<version>/bin/go and uses that for all make commands.
We use the name GNUmakefile instead of Makefile because that will prevent people from using a non-GNU make command to run make commands.

This doesn't prevent anyone from doing things using their normal style but it adds the following benefits for everyone:

• make test just works in almost any environment
• You can clone the repo and run make test without needing anything installed (except make, curl and bash).
• Test with any go version using: make test GO-VERSION=1.23.4
• To get a shell with a particular go version installed and ready: make shell GO-VERSION=1.23.4

[ingydotnet]

This should work out for anyone on Linux / Mac if they have git, bash, curl and make installed. (minimal)
It self-installs the Makefile parts and then self-installs Go and sets the environment to run it, without affecting the user's system in any other way.

Example output of the self-installing make commands:

$ make distclean
rm -f -r .cache
$ 
$ make test
Installing 'go' locally
curl -Ls https://go.dev/dl/go1.24.3.linux-amd64.tar.gz > /home/ingy/src/go-yaml/.cache/.local/cache/go1.24.3.linux-amd64.tar.gz
tar -C /home/ingy/src/go-yaml/.cache/.local -xzf /home/ingy/src/go-yaml/.cache/.local/cache/go1.24.3.linux-amd64.tar.gz
mv /home/ingy/src/go-yaml/.cache/.local/go /home/ingy/src/go-yaml/.cache/.local/go-1.24.3
touch /home/ingy/src/go-yaml/.cache/.local/go-1.24.3/bin/go

go test 
OK: 50 passed
PASS
ok  	go.yaml.in/yaml/v3	2.236s
$ 
$ make test
go test 
OK: 50 passed
PASS
ok  	go.yaml.in/yaml/v3	2.245s
$ 

[ingydotnet]

I need to finish the work for macOS, then I'll take it out of draft.

[ingydotnet]

This is working well on Linux and Mac now.

Ready for review.

[ingydotnet]

Had to remove a file. Ready for real now.

[ingydotnet]

Updated and ready for review. Added support for make test v=1. Only installs local go is no go is installed or GO-VERSION=... is used.

[stefanprodan]

LGTM