Ports: Add default zshrc

Previosly we had a very messed up PS1 as the Shell PROMPT is not
unset correctly.

We now provide a default `zshrc` file for the system that uses
sane values for basic categories like aliases, autocompletion and
history management to make the port more usable. It also forces
the prompt to be the default zsh one.

Ports/zsh/package.sh

@@ -13,3 +13,7 @@ pre_configure() {
 post_configure() {
     run_replace_in_file "s/define HAVE_PRCTL 1/undef HAVE_PRCTL/" config.h
 }
+
+post_install() {
+    cp "${PORT_META_DIR}/zshrc" "${SERENITY_INSTALL_ROOT}/etc/"
+}