parser: rename main function to __rayoko_main

examples/hello.ry

@@ -30,5 +30,5 @@ fn add(a: i32, b: i32) i32 {
 }
 
 fn main() i32 {
-    return 0;
+    return 1;
 }

src/entry.c

@@ -0,0 +1,3 @@
+int main(void) {
+    __rayoko_main();
+}

src/parsers.zig

@@ -464,7 +464,13 @@ pub const Parser = struct {
             method = try self.parsePreMethod();
         }
 
-        const name = try self.consumeSingle(.Identifier);
+        const orig_name = try self.consumeSingle(.Identifier);
+
+        const name = if (std.mem.eql(u8, orig_name.lexeme, "main")) blk: {
+            break :blk try self.mkToken(.Identifier, "__rayoko_main", orig_name.line);
+        } else blk: {
+            break :blk orig_name;
+        };
 
         self.setErrContext("function {}", name.lexeme);