v.parser: change body_start_pos to function declaration's lcbr, resolve #13642 (#13646)

vlib/v/parser/fn.v

@@ -466,10 +466,10 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	// Body
 	p.cur_fn_name = name
 	mut stmts := []ast.Stmt{}
-	body_start_pos := p.peek_tok.pos()
+	body_start_pos := p.tok.pos()
 	if p.tok.kind == .lcbr {
 		if language != .v && language != .js {
-			p.error_with_pos('interop functions cannot have a body', p.tok.pos())
+			p.error_with_pos('interop functions cannot have a body', body_start_pos)
 		}
 		p.inside_fn = true
 		p.inside_unsafe_fn = is_unsafe

vlib/v/parser/tests/fn_body_start_pos.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/fn_body_start_pos.vv:1:14: error: functions with type only args can not have bodies
+    1 | fn func(int) {}
+      |              ^

vlib/v/parser/tests/fn_body_start_pos.vv

@@ -0,0 +1 @@
+fn func(int) {}

vlib/v/parser/tests/fn_type_only_args_no_body.out

@@ -1,6 +1,5 @@
-vlib/v/parser/tests/fn_type_only_args_no_body.vv:2:1: error: functions with type only args can not have bodies 
+vlib/v/parser/tests/fn_type_only_args_no_body.vv:1:14: error: functions with type only args can not have bodies
     1 | fn test(int) {
+      |              ^
     2 | }
-      | ^
+    3 |
-    3 | 
-    4 | fn main() {

vlib/v/parser/tests/fn_type_only_args_unknown_name.out

@@ -1,6 +1,5 @@
-vlib/v/parser/tests/fn_type_only_args_unknown_name.vv:2:1: error: functions with type only args can not have bodies 
+vlib/v/parser/tests/fn_type_only_args_unknown_name.vv:1:16: error: functions with type only args can not have bodies
     1 | fn test(param) {
+      |                ^
     2 | }
-      | ^
+    3 |
-    3 | 
-    4 | fn main() {