parser: error for consts in fns

vlib/v/checker/tests/const_define_in_function_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/const_define_in_function_err.v:2:2: error: const can only be defined at the top level (outside of functions)
+    1 | fn main() {
+    2 |     const (a = 1)
+      |     ~~~~~
+    3 |     println(a)
+    4 | }

vlib/v/checker/tests/const_define_in_function_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+	const (a = 1)
+	println(a)
+}

vlib/v/parser/parser.v

@@ -504,6 +504,9 @@ pub fn (mut p Parser) stmt() ast.Stmt {
 			}
 		}
 		else {
+			if p.tok.kind == .key_const {
+				p.error_with_pos('const can only be defined at the top level (outside of functions)', p.tok.position())
+			}
 			epos := p.tok.position()
 			return ast.ExprStmt{
 				expr: p.expr(0)