Remove support for <style> elements in default template engine.

[theengineear]

Now that the “Import Attributes” TC39 proposal has been accepted, browsers are allowing developers to import instantiated CSSStyleSheet objects from .css files directly. This obviates the need to inline a <style> tag in a template — instead, you can just move it to a .css file.

Since (1) the rules to support <style> tags are specialized and (2) it’s preferable to externalize style sheets anyhow — this seems like an obvious choice to restrict usage of <style> within the default template engine.