Make include order follow Google/LLVM/Lakos guidelines

.clang-format

@@ -7,7 +7,7 @@
   BreakBeforeBraces: Attach
   ColumnLimit: '100'
   ConstructorInitializerAllOnOneLineOrOnePerLine: 'true'
-  IncludeBlocks: Regroup
+  IncludeBlocks: Preserve
   IndentPPDirectives: AfterHash
   IndentWidth: '2'
   NamespaceIndentation: All

source/greeter.cpp

@@ -1,6 +1,7 @@
-#include <fmt/format.h>
 #include <greeter/greeter.h>
 
+#include <fmt/format.h>
+
 using namespace greeter;
 
 Greeter::Greeter(std::string _name) : name(std::move(_name)) {}

test/source/greeter.cpp

@@ -1,7 +1,8 @@
-#include <doctest/doctest.h>
 #include <greeter/greeter.h>
 #include <greeter/version.h>
 
+#include <doctest/doctest.h>
+
 #include <string>
 
 TEST_CASE("Greeter") {