parser: fix parsing interface methods with varargs (#8229)

vlib/v/tests/interface_variadic_test.v

@@ -0,0 +1,25 @@
+interface Element {
+	method(params ...f64) string
+}
+
+struct Foo {}
+
+fn (f &Foo) method(params ...f64) string {
+	return params.str()
+}
+
+fn test_variadic_array_decompose() {
+	mut a := []Element{}
+	a << Foo{}
+
+	input := [0.0, 1.0]
+	assert a[0].method(...input) == '[0, 1]'
+	assert a[0].method(...[0.0, 1.0]) == '[0, 1]'
+}
+
+fn test_variadic_multiple_args() {
+	mut a := []Element{}
+	a << Foo{}
+
+	assert a[0].method(0.0, 1.0) == '[0, 1]'
+}