Add fmt submodule.

.gitmodules

@@ -55,3 +55,6 @@
 [submodule "third_party/utfcpp"]
 	path = third_party/utfcpp
 	url = https://github.com/xenia-project/utfcpp.git
+[submodule "third_party/fmt"]
+	path = third_party/fmt
+	url = https://github.com/fmtlib/fmt.git

third_party/fmt.lua

@@ -0,0 +1,17 @@
+group("third_party")
+project("fmt")
+  uuid("b9ff4b2c-b438-42a9-971e-e0c19a711a13")
+  kind("StaticLib")
+  language("C++")
+  links({
+  })
+  defines({
+    "_LIB",
+  })
+  includedirs({
+    "fmt/include",
+  })
+  files({
+    "fmt/src/format.cc",
+    "fmt/src/os.cc"
+  })