Implement updated HTML language inheritance rules.

This makes two changes to match the HTML spec:
* only honor the unnamespaced lang attribute on HTML and SVG elements
* have <slot> elements in a shadow tree inherit language from the shadow
  host

Note that an update to the HTML specification regarding the SVG elements
case is proposed in whatwg/html#9882, but
https://www.w3.org/TR/SVG/struct.html#LangAttribute already defines it.

These changes match the related changes to the dir attribute, but use a
separate feature flag as a killswitch, since they are separable.

Fixed: 1490711
Bug: 576815
Change-Id: I6462181e6d4433aa1dcc9a2c42e5c12754830e52

html/dom/elements/global-attributes/lang-attribute.window.js

@@ -0,0 +1,16 @@
+test(() => {
+  const container = document.createElement("div");
+  document.body.append(container);
+  container.setAttribute("lang", "en-CA");
+
+  const child = document.createElementNS("div", "test");
+  container.append(child);
+  child.setAttribute("lang", "en-NZ");
+
+  assert_true(container.matches(":lang(en-CA)"), "container matches en-CA");
+  assert_true(child.matches(":lang(en-CA)"), "child matches en-CA");
+  assert_false(container.matches(":lang(en-NZ)"), "container does not match en-NZ");
+  assert_false(child.matches(":lang(en-NZ)"), "child does not match en-NZ");
+
+  container.remove();
+}, "unnamespaced lang attribute only works on elements in the HTML namespace");