parser: check error for script mode that define main function (fix #14467) (#14469)

Author: yuyi98

vlib/v/checker/tests/main_and_script_err.out

@@ -693,6 +693,10 @@ pub fn (mut p Parser) top_stmt() ast.Stmt {
 					p.script_mode = true
 					p.script_mode_start_token = p.tok
 
+					if p.table.known_fn('main.main') {
+						p.error('function `main` is already defined, put your script statements inside it')
+					}
+
 					p.open_scope()
 					mut stmts := []ast.Stmt{}
 					for p.tok.kind != .eof {

vlib/v/parser/parser.v

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/main_and_script_err.vv:4:1: error: function `main` is already defined, put your script statements inside it
+    2 |     println('main')
+    3 | }
+    4 | println('out')
+      | ~~~~~~~