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.

tests/plugins/mod.rs

@@ -1,4 +1,3 @@
-mod cache_file;
 mod config;
 mod core_inc;
 mod custom_values;
@@ -6,5 +5,6 @@ mod env;
 mod formats;
 mod nu_plugin_nu_example;
 mod register;
+mod registry_file;
 mod stream;
 mod stress_internals;