parser: proper error on fn decl in script mode (#7680)

vlib/v/parser/fn.v

@@ -424,6 +424,11 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 fn (mut p Parser) anon_fn() ast.AnonFn {
 	pos := p.tok.position()
 	p.check(.key_fn)
+	if p.pref.is_script && p.tok.kind == .name {
+		p.error_with_pos('function declarations in script mode should be before all script statements',
+			p.tok.position())
+		return ast.AnonFn{}
+	}
 	p.open_scope()
 	// TODO generics
 	args, _, is_variadic := p.fn_args()

vlib/v/parser/tests/invalid_fn_decl_script_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/invalid_fn_decl_script_err.vv:3:4: error: function declarations in script mode should be before all script statements
+    1 | mynum := 10
+    2 |
+    3 | fn main() {
+      |    ~~~~
+    4 |     println(mynum)
+    5 | }

vlib/v/parser/tests/invalid_fn_decl_script_err.vv

@@ -0,0 +1,5 @@
+mynum := 10
+
+fn main() {
+	println(mynum)
+}