👩‍🎓 Differentiate behavior of newcommand, renewcommand, providecommand

[fwkoch]

Previously, providecommand was not supported and renewcommand did not care if the macro was already defined. Now:

• newcommand: warns if macro is already defined (still overrides the previous command, to maintain previous behavior with MyST)
• renewcommand: warns if macro is not already defined (either way, it sets the new command)
• providecommand: never warns - if macro is not defined, it sets the new command; if macro is defined, it does nothing