patch 8.1.1224: MS-Windows: cannot specify font weight

Problem:    MS-Windows: cannot specify font weight.
Solution:   Add the "W" option to 'guifont'. (closes #4309)  Move GUI font
            explanation out of options.txt.
diff --git a/src/os_mswin.c b/src/os_mswin.c
index 48be0de..503d3d7 100644
--- a/src/os_mswin.c
+++ b/src/os_mswin.c
@@ -2992,6 +2992,9 @@
 	    case L'w':
 		lf->lfWidth = points_to_pixels(p, &p, FALSE, (long_i)printer_dc);
 		break;
+	    case L'W':
+		lf->lfWeight = wcstol(p, &p, 10);
+		break;
 	    case L'b':
 		lf->lfWeight = FW_BOLD;
 		break;