Allow lowercase letters for array tool pin numbering

The ARRAY_AXIS class now accepts lowercase letters (a-z) as input for
alphabetic numbering schemes and preserves that case in the output.
This allows users to create pin numbers like a1, b1, c1 which are
commonly used in DIN 41612 and other backplane connectors.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/22787

include/array_axis.h

@@ -125,6 +125,9 @@ private:
 
     /// Skip every 'n' numbers.
     int m_step;
+
+    /// Output lowercase letters when true (for alphabetic types).
+    bool m_useLowercase;
 };
 
 #endif // ARRAY_AXIS__H