i was writing #8148 and came to the "_Test + Formatting_" section of the
PR template.
i felt the developer instructions could be wrapped up in a common easy
to use format, e.g. a `Makefile`, to be used with a few-words command
only, e.g. `make fmt` or `make clippy`, instead of the long commands in
the PR template 🤔
> **Note**
> in case you guys do not want to add a `Makefile` to the `nushell`
source, that PR can be discarded 😌
# Description
this PR
- adds the few rules from the PR template to a new `Makefile`
- replaces the instructions in the PR template from the full commands to
the new `make` rules
# User-Facing Changes
- _none for the regular user_
- i believe easier to test PRs for the developer, allowing one not to
realy on knowing the long commands or using the shell history to run
them again 😋
# Tests + Formatting
_nothing to test_
# After Submitting
maybe mention that in `CONTRIBUTING.md`?
