Add lexy package (#1156)

* add lexy package

* simplify xmake.lua

* typo

packages/l/lexy/xmake.lua

@@ -0,0 +1,15 @@
+package("lexy")
+    set_kind("library", {headeronly = true})
+    set_homepage("https://lexy.foonathan.net")
+    set_description("C++ parsing DSL")
+
+    add_urls("https://github.com/foonathan/lexy.git")
+    add_versions("2022.03.21", "10342c6b1a03cbc6254c64064b419799a7993e0e")
+
+    on_install(function (package)
+        os.cp("include", package:installdir())
+    end)
+
+    on_test(function (package)
+        assert(package:has_cxxincludes("lexy/dsl.hpp", {configs = {languages = "c++17"}}))
+    end)