Add info for contributors

[laniakea64]

Tag #75

Add recipe to verify that (most) test cases are runnable justfiles

... and explicitly list the exceptions.

The primary purpose of vim-just is to highlight correct justfile syntax. And our highlighting is not strictly limited to documented forms of just syntaxes, we try highlight how just actually behaves. So having most test cases be valid, runnable justfiles is helpful in development.

This recipe hints to contributors that keeping most test cases actually runnable is of value to the project, and the exception list documents which ones are not intended to be runnable.

Add some explanatory comments to syntax/just.vim

Document a few things that maybe subtle, to help awareness for contributors looking at those areas of the code.

[NoahTheDuke]

I didn't write any of the test cases with the intention that they'd be runnable, just that they'd verify a specific shape->highlight. I think that keeping them generally runnable (which is to say, they don't error when you run them to print the list of recipes) is smart tho.