Update imgui to v1.8.1 (#287)

packages/i/imgui/xmake.lua

@@ -6,6 +6,7 @@ package("imgui")
     add_urls("https://github.com/ocornut/imgui/archive/$(version).tar.gz",
              "https://github.com/ocornut/imgui.git")
 
+    add_versions("v1.81", "f7c619e03a06c0f25e8f47262dbc32d61fd033d2c91796812bf0f8c94fca78fb")
     add_versions("v1.80", "d7e4e1c7233409018437a646680316040e6977b9a635c02da93d172baad94ce9")
     add_versions("v1.79", "f1908501f6dc6db8a4d572c29259847f6f882684b10488d3a8d2da31744cd0a4")
     add_versions("v1.75", "1023227fae4cf9c8032f56afcaea8902e9bfaad6d9094d6e48fb8f3903c7b866")