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/class/function_param.carbon

@@ -20,6 +20,6 @@ fn GetX(p: Point) -> i32 {
   return p.x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return GetX({.x = 1, .y = 2}) - 1;
 }