Rename plugin cache file ⇒ plugin registry file (#12634)

# Description
So far this seems like the winner of my poll on what the name should be.
I'll take this off draft once the poll expires, if this is indeed the
winner.

src/command.rs

@@ -392,13 +392,13 @@ impl Command for Nu {
                 .named(
                     "plugin-config",
                     SyntaxShape::Filepath,
-                    "start with an alternate plugin cache file",
+                    "start with an alternate plugin registry file",
                     None,
                 )
                 .named(
                     "plugins",
                     SyntaxShape::List(Box::new(SyntaxShape::Filepath)),
-                    "list of plugin executable files to load, separately from the cache file",
+                    "list of plugin executable files to load, separately from the registry file",
                     None,
                 )
         }