parser: fix the position of the multi variable += assign error (#7584)

vlib/v/parser/assign.v

@@ -106,7 +106,7 @@ fn (mut p Parser) partial_assign_stmt(left []ast.Expr, left_comments []ast.Comme
 		for r in right {
 			has_cross_var = p.check_cross_variables(left, r)
 			if op !in [.assign, .decl_assign] {
-				p.error('unexpected $op.str(), expecting := or = or comma')
+				p.error_with_pos('unexpected $op.str(), expecting := or = or comma', pos)
 				return ast.Stmt{}
 			}
 			if has_cross_var {

vlib/v/parser/tests/multi_argumented_assign_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/multi_argumented_assign_err.vv:3:10: error: unexpected +=, expecting := or = or comma
+    1 | fn main() {
+    2 |     mut a, mut b, mut c := 0,1, 2 
+    3 |     a, b, c += 1, 2, 4
+      |             ~~
+    4 |     println('$a $b $c')
+    5 | }

vlib/v/parser/tests/multi_argumented_assign_err.vv

@@ -0,0 +1,5 @@
+fn main() {
+	mut a, mut b, mut c := 0,1, 2 
+	a, b, c += 1, 2, 4
+	println('$a $b $c')
+}