feat: Dynamic start print actions

Revamp the current logic to hook into the START_PRINT macro and
implement the same dynamic approach as END_PRINT for Gcode macros
starting with `_START_PRINT_ACTION`, again verifying existence
before calling the macro.

macros/base/start_print.cfg

@@ -140,6 +140,13 @@ gcode:
 
     # Here is the core of the START_PRINT were we get the startprint_actions variable
     # to do the procedure in the correct order for the configured probe (or user custom override)
+    #
+    # Custom actions can be called by defining a gcode macro as in the following example and
+    # adding `custom1` to the startprint_actions override config.
+    # [gcode_macro _START_PRINT_ACTION_CUSTOM1]
+    # gcode:
+    #   ## Your custom code here
+    #
     {% set sp_actions = printer["gcode_macro _USER_VARIABLES"].startprint_actions %}
     {% for action in sp_actions %}
         {% if action == "bed_soak" %}
@@ -169,7 +176,11 @@ gcode:
         {% elif action == "custom3" %}
             _MODULE_CUSTOM3 {rawparams}
         {% else %}
-            { action_raise_error("Unknown module called in START_PRINT! Please verify your startprint_actions variable override!") }
+            {% if "_START_PRINT_ACTION_%s" % (action|upper) in printer.gcode.commands %}
+                _START_PRINT_ACTION_{action|upper} {rawparams}
+            {% else %}
+                { action_raise_error("Unknown module %s called in START_PRINT! Please verify your startprint_actions variable override!" % (action)) }
+            {% endif %}
         {% endif %}
     {% endfor %}