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/global_variable/write.carbon

@@ -15,7 +15,7 @@ package ExecutableSemanticsTest api;
 
 var zero: i32 = 1;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   zero = 0;
   return zero;
 }