checker: prompt error on script expression while inside a file with main (#6400)

vlib/v/checker/checker.v

@@ -191,6 +191,9 @@ fn (mut c Checker) check_file_in_main(file ast.File) bool {
 			}
 			ast.FnDecl {
 				if stmt.name == 'main.main' {
+					if has_main_fn {
+						c.error('function `main` is already defined', stmt.pos)
+					}
 					has_main_fn = true
 					if stmt.is_pub {
 						c.error('function `main` cannot be declared public', stmt.pos)

vlib/v/checker/tests/main_and_script_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/main_and_script_err.vv:1:1: error: function `main` is already defined
+    1 | fn main() {
+      | ^
+    2 |     println('main')
+    3 | }

vlib/v/checker/tests/main_and_script_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+	println('main')
+}
+println('out')