checker: make sure `main` has no args and doesn't return

vlib/v/checker/checker.v

@@ -115,6 +115,12 @@ fn (mut c Checker) check_file_in_main(file ast.File) bool {
 					if it.is_pub {
 						c.error('function `main` cannot be declared public', it.pos)
 					}
+					if it.args.len > 0 {
+						c.error('function `main` cannot have arguments', it.pos)
+					}
+					if it.return_type != table.void_type {
+						c.error('function `main` cannot return values', it.pos)
+					}
 				} else {
 					if it.is_pub {
 						c.warn('function `$it.name` $no_pub_in_main_warning', it.pos)

vlib/v/checker/tests/main_args_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/main_args_err.v:1:1: error: function `main` cannot have arguments
+    1| fn main(a string) {
+       ~~~~~~~~~~~~~~~~~
+    2|     println(a)
+    3| }

vlib/v/checker/tests/main_args_err.vv

@@ -0,0 +1,3 @@
+fn main(a string) {
+	println(a)
+}

vlib/v/checker/tests/main_return_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/main_return_err.v:1:1: error: function `main` cannot return values
+    1| fn main() f64 {
+       ~~~~~~~~~~~~~
+    2|     return 1.23
+    3| }

vlib/v/checker/tests/main_return_err.vv

@@ -0,0 +1,3 @@
+fn main() f64 {
+	return 1.23
+}