feat: Split _path_add into _path_append and _path_prepend

_path_add only ever appended to PATH, which didn't fit profile.d/krew
and profile.d/anyenv (they prepend, to take priority). Rename it to
_path_append for clarity and add _path_prepend alongside it, so both
orderings get the same dedup guard when needed — krew and anyenv
aren't converted yet, kept as plain PATH concatenation until then.

rc.d/rvm

@@ -5,4 +5,4 @@
 [[ -r "${HOME}/.rvm/scripts/completion" ]] && source "${HOME}/.rvm/scripts/completion"
 
 # Add RVM to PATH for scripting
-[[ -r "${HOME}/.rvm/bin" ]] && _path_add "${HOME}/.rvm/bin"
+[[ -r "${HOME}/.rvm/bin" ]] && _path_append "${HOME}/.rvm/bin"