Add $INSUNITS header variable to DXF export

The DXF exporter was missing the $INSUNITS header variable, which tells
CAD software what physical units the drawing coordinates represent. Without
it, other programs have no reliable way to determine the coordinate scale.

Add $INSUNITS to the HEADER section alongside the existing $MEASUREMENT
variable. When exporting in millimeters, $INSUNITS is set to 4; when
exporting in inches, it is set to 1.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/23181

(cherry picked from commit c0a6a139cf)

common/plotters/DXF_plotter.cpp

@@ -141,12 +141,14 @@ void DXF_PLOTTER::SetUnits( DXF_UNITS aUnit )
     case DXF_UNITS::MM:
         m_unitScalingFactor = 0.00254;
         m_measurementDirective = 1;
+        m_insUnits = 4;
         break;
 
     case DXF_UNITS::INCH:
     default:
         m_unitScalingFactor = 0.0001;
         m_measurementDirective = 0;
+        m_insUnits = 1;
     }
 }
 
@@ -219,6 +221,10 @@ bool DXF_PLOTTER::StartPlot( const wxString& aPageNumber )
             "$MEASUREMENT\n"
             "  70\n"
             "%u\n"
+            "  9\n"
+            "$INSUNITS\n"
+            "  70\n"
+            "%u\n"
             "  0\n"
             "ENDSEC\n"
             "  0\n"
@@ -313,7 +319,7 @@ bool DXF_PLOTTER::StartPlot( const wxString& aPageNumber )
             "-0.2\n"
             "  0\n"
             "ENDTAB\n",
-             GetMeasurementDirective() );
+             GetMeasurementDirective(), GetInsUnits() );
 
     // Text styles table
     // Defines 4 text styles, one for each bold/italic combination