Rename the LINE_PURGE and SMART_PARK macros to be hidden

[pellcorp]

There is no benefit to having these macros be visible in fluidd or mainsail

It will affect people who have overridden LINE_PURGE the old way, which is to remove the Line_Purge.cfg and add their own LINE_PURGE, but we can easily maintain backwards compatibility with:

{% if printer["gcode_macro _SAF_LINE_PURGE"] != null %}
        _SAF_LINE_PURGE {rawparams}
      {% elif printer["gcode_macro LINE_PURGE"] != null  %}
        LINE_PURGE # support users who overridden LINE_PURGE the old way
     {% else %}
        _LINE_PURGE
      {% endif %}

We might consider doing the same for SMART_PARK too honestly