fix(xtext): move hexcolor_parse to its own file for the standalone xtext tests

master's new xtext_tests target compiles xtext.c on its own with only
GTK linked, so hexcolor_parse cannot live in util.c. Keep it in a
dependency-free hexcolor.c that both test targets compile, and register
it in the Visual Studio project for the Windows build.

Claude-Session: https://claude.ai/code/session_01Jxd4AknGzdAENADWAGHCzx

src/common/hexcolor.c

@@ -0,0 +1,42 @@
+/* ZoiteChat
+ * Parsing of \004 (hex color) formatting codes: RRGGBB[,RRGGBB].
+ * Kept free of other application dependencies so the xtext widget tests can
+ * compile it on its own.
+ * Distributed under GPL-2.0-or-later; see COPYING. */
+
+#include "hexcolor.h"
+
+static int
+hexcolor_component (const char *s, int avail, guint32 *rgb)
+{
+	int i;
+	guint32 value = 0;
+
+	if (avail < 6)
+		return FALSE;
+	for (i = 0; i < 6; i++)
+	{
+		if (!g_ascii_isxdigit (s[i]))
+			return FALSE;
+		value = (value << 4) | g_ascii_xdigit_value (s[i]);
+	}
+	if (rgb)
+		*rgb = value;
+	return TRUE;
+}
+
+int
+hexcolor_parse (const char *s, int avail, guint32 *fg, guint32 *bg, int *has_bg)
+{
+	if (has_bg)
+		*has_bg = FALSE;
+	if (!hexcolor_component (s, avail, fg))
+		return 0;
+	if (avail >= 7 && s[6] == ',' && hexcolor_component (s + 7, avail - 7, bg))
+	{
+		if (has_bg)
+			*has_bg = TRUE;
+		return 13;
+	}
+	return 6;
+}