checker: make the no-body main function a checker error (#8211)

vlib/v/checker/checker.v

@@ -246,6 +246,9 @@ fn (mut c Checker) check_file_in_main(file ast.File) bool {
 					if stmt.return_type != table.void_type {
 						c.error('function `main` cannot return values', stmt.pos)
 					}
+					if stmt.no_body {
+						c.error('function `main` must declare a body', stmt.pos)
+					}
 				} else {
 					for attr in stmt.attrs {
 						if attr.name == 'console' {

vlib/v/checker/tests/main_no_body_err.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/main_no_body_err.vv:1:1: error: function `main` must declare a body
+    1 | fn main()
+      | ~~~~~~~~~

vlib/v/checker/tests/main_no_body_err.vv

@@ -0,0 +1 @@
+fn main()