testing: Add manual instance test for Style

PR nushell#43 introduced a pub(crate) field in Style which broke the
intended API (See: Issue nushell#46). Introduce a new test which will fail in those
cases since it won't be able to initialize pub(crate) fields.

Inspired by: nushell#46 (comment)

tests/style.rs

@@ -0,0 +1,28 @@
+use nu_ansi_term::Style;
+
+#[test]
+fn manual_instance_style() {
+    let s = Style { ..Style::default() };
+    assert_eq!(Style::default(), s);
+
+    let s = Style {
+        is_underline: false,
+        ..Style::default()
+    };
+    assert_eq!(Style::default(), s);
+
+    let s = Style {
+        foreground: None,
+        background: None,
+        is_bold: false,
+        is_dimmed: false,
+        is_italic: false,
+        is_underline: false,
+        is_blink: false,
+        is_reverse: false,
+        is_hidden: false,
+        is_strikethrough: false,
+        with_reset: false,
+    };
+    assert_eq!(Style::default(), s);
+}