Add Epson test page: text sizes, EAN-13, QR, and a raster image

ReceiptBuilder#image implements GS v 0 raster printing (no image library
needed for the demo -- a procedural 8px checkerboard proves it end to
end). Skipped attempting the printer's real-time clock: unsure the
TM-T20II actually has a battery-backed RTC (that's usually an option on
higher-end models), not worth guessing at an undocumented command.

views/epson.erb

@@ -15,6 +15,10 @@
           <button class="btn btn-secondary" type="submit">Print</button>
         </form>
 
+        <form action="/epson/test-print" method="POST" class="mt-3">
+          <button class="btn btn-outline-secondary" type="submit">Print test page (sizes, barcode, QR, image)</button>
+        </form>
+
         <hr class="mt-4">
 
         <h5>Free text (ESC/POS mnemonic notation)</h5>