Implement NavigatorLanguage

[jdm]

Spec: https://html.spec.whatwg.org/multipage/webappapis.html#navigatorlanguage (and add Navigator implements NavigatorLanguage;)
Code: components/script/dom/webidls/Navigator.webidl, components/script/dom/navigator.rs
Tests:

• ./mach test-wpt tests/wpt/html/dom/interfaces.html
• ./mach test-wpt tests/wpt/web-platform-tests/html/webappapis/system-state-and-capabilities/the-navigator-object/navigatorlanguage.html
• Follow the documentation to adjust for tests that now pass.

The lack of this feature is makes clicking links on Twitter break (see #9870). Let's just hardcode this as en-US right now.

[creativcoder]

I'll take that, will help me get familiar with dom parts.

[jdm]

Great!

[creativcoder]

I am getting an error from the CodegenRust.pyfor the languages attribute, which returns a StringArray.
Maybe CodegenRust.py does not have type declarations for DOMString[] types:
Here's some stack trace:
File "/home/creativcoder/Desktop/github/servo/components/script/dom/bindings/codegen/CodegenRust.py", line 1310, in getRetvalDeclarationForType returnType) TypeError: Don't know how to declare return value for StringArray

[Ms2ger]

Start with just language for now; we can do languages later when whatwg/html#863 is fixed and we support FrozenArray.