fmt: fix formatting array decomposition (#7835)

vlib/v/fmt/fmt.v

@@ -1140,6 +1140,7 @@ pub fn (mut f Fmt) expr(node ast.Expr) {
 		}
 		ast.ArrayDecompose {
 			f.expr(node.expr)
+			f.write('...')
 		}
 	}
 }

vlib/v/fmt/tests/array_decomposition_keep.vv

@@ -0,0 +1,8 @@
+fn varargs(a ...int) {
+	println(a)
+}
+
+fn main() {
+	a := [1, 2, 3]
+	varargs(a...)
+}