Rename message functions file for new notice API

Part of #4034
Initial commit of notices.php template

Part of #4034
Introduce new wc_notice_count() function

To replace wc_message_count() & wc_error_count()

Part of #4034
Introduce new wc_add_notice() function

To replace wc_add_message() & wc_add_error() functions.

Part of #4034
Introduce wc_clear_notices() function

To replace wc_clear_messages() & wc_clear_errors() functions

Part of #4034
Introduce new wc_print_notices() function

To replace wc_print_messages() function.

Part of #4034
Update calls to wc_add_error() function

Part of #4034
Update calls to wc_add_message() function

Part of #4034
Update calls to wc_error_count() function

Part of #4034
Update calls to wc_print_messages() function

Part of #4034
Update calls to wc_clear_messages() function

Part of #4034
Update calls to wc_message_count() function

Part of #4034
Move notice templates into /notices/notice.php
Store all notices in 'wc_notices' array

With 'error', 'success' & 'notice' array keys
Introduce new wc_get_notices() function

To replace the deprecated Woocommerce->get_messages() and
Woocommerce->get_errors() functions
Introduce new wc_print_notice() function
Use wc_print_notice() for checkout notices
fixup! Store all notices in 'wc_notices' array
Use $messages variable for all notice templates
Use new notice template for Shipping Updated

For #4034