Add initial design files for 27 MHz clock counter

- Created tangnano20k.cst to define I/O locations for clock, button, and LEDs.
- Implemented top.v module to instantiate the counter with clock and button inputs, and 4-bit count output.

Introduction/counter/tangnano20k.cst

@@ -0,0 +1,14 @@
+# Horloge 27 MHz (souvent sur PIN4)
+IO_LOC "clk" 4;
+IO_PORT "clk" IO_TYPE=LVCMOS33;
+
+# Bouton KEY1 (utilisé ici comme btn1 pour reset)
+IO_LOC "btn1" 88;
+IO_PORT "btn1" IO_TYPE=LVCMOS33;
+
+# LEDs (pour afficher le compteur)
+IO_LOC "count[0]" 15;
+IO_LOC "count[1]" 16;
+IO_LOC "count[2]" 17;
+IO_LOC "count[3]" 18;
+IO_PORT "count[3:0]" IO_TYPE=LVCMOS33;