Drop `which` in favor of the more standard `command -v`

bin/mdvimb

@@ -140,7 +140,7 @@ main() {
     MdFile=$1; shift
     debug -v MDVIMB__CONVERTER MDVIMB__CONFIG_HOME MDVIMB__STYLE
     debug -v MdFile converter NoOpen Wrap Style ConfigHome
-    converterpath=$(which "$converter") \
+    converterpath=$(command -v "$converter") \
      || die "converter not available: $converter"
     debug -v converterpath
     think "using converter: $converter"