add utf8.h (#1715)

packages/u/utf8.h/xmake.lua

@@ -0,0 +1,15 @@
+package("utf8.h")
+    set_kind("library", {headeronly = true})
+    set_homepage("https://github.com/sheredom/utf8.h")
+    set_description("single header utf8 string functions for C and C++")
+
+    add_urls("https://github.com/sheredom/utf8.h.git")
+    add_versions("2022.07.04", "4e4d828174c35e4564c31a9e35580c299c69a063")
+
+    on_install(function (package)
+        os.cp("*.h", package:installdir("include"))
+    end)
+
+    on_test(function (package)
+        assert(package:has_cfuncs("utf8casecmp", {includes = "utf8.h"}))
+    end)