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/common.vcxproj

@@ -43,6 +43,7 @@
     <ClInclude Include="typedef.h" />
     <ClInclude Include="url.h" />
     <ClInclude Include="userlist.h" />
+    <ClInclude Include="hexcolor.h" />
     <ClInclude Include="util.h" />
     <ClInclude Include="zoitechat-plugin.h" />
     <ClInclude Include="zoitechat.h" />
@@ -79,6 +80,7 @@
     <ClCompile Include="tree.c" />
     <ClCompile Include="url.c" />
     <ClCompile Include="userlist.c" />
+    <ClCompile Include="hexcolor.c" />
     <ClCompile Include="util.c" />
     <ClCompile Include="zoitechat.c" />
     <ClCompile Include="theme-service.c" />