Replace mise-activate with simple wrapper scripts and ENV vars

The prompting for this change is to use mise within ksh/oksh without
having to rely on zsh (such as `zsh -c "mise activate zsh"` or
otherwise). I kind of like the simplicity of this, and then I can still
easily change versions as needed in a given project by sourcing a file
which exports different vars for target language versions, or I can just
do manual export commands from the shell.

src_files/.local/bin/ruby

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+mise exec ruby@$MISE_RUBY_VERSION -- ruby "$@"