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/fe-gtk/meson.build

@@ -280,7 +280,7 @@ test('Emoji Font Tests', emoji_font_tests,
 )
 
 xtext_tests = executable('xtext_tests',
-  'tests/test-xtext.c',
+  ['tests/test-xtext.c', '../common/hexcolor.c'],
   include_directories: [config_h_include],
   dependencies: [gtk_dep],
 )