Rename main to Main for Carbon style consistency (#939)

Main() is more consistency with Carbon's naming guidelines. C# offers some precedent: https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/program-structure/main-command-line

executable_semantics/testdata/function/empty_params.carbon

@@ -14,7 +14,7 @@ package ExecutableSemanticsTest api;
 // Test empty parameters and return type
 fn f() { }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   f();
   return 0;
 }