remove dependence on environ; remove \makeatletter/\makeatother in minim-mp.sty

The environ package is no longer necessary in LaTeX because the kernel provides the b argument spec for \NewDocumentEnvironment which accomplishes the same thing, so this PR removes that dependency and changes the corresponding commands.

I also removed \makeatletter and \makeatother since these are unnecessary in a package file. See e.g. https://tex.stackexchange.com/a/62586/208544 for explanation of this.

Nice package, by the way!