Allow graceful degradation into textarea when there's no JavaScript / support for code-input

[WebCoder49]

I'm thinking:

<code-input language="javascript" placeholder="JavaScript code, highlighted"><textarea data-code-input-fallback placeholder="JavaScript code, not highlighted">// This is a demo file
console.log("Hello, World!");
</textarea></code-input>

If this syntax (including the data-code-input-fallback) conflicts with anyone's usage so far I'll be incredibly surprised; if HTML is written inside a code-input and contains a textarea[data-code-input-fallback], the <s can be replaced with <s.

Very little code: just detect the textarea, transfer its value to the code-input value if it's present, and transfer textareaSyncAttributes to the code-input if the code-input doesn't already have them. CSS must be updated to override invisible text in textarea[data-code-input-fallback] - maybe always make it black on white.

This is useful because code-input can be used with HTML forms and no extra JavaScript.

[WebCoder49]

For the record, this fallback isn't fully earliest-standards-compliant with the unknown <code-input> tags surrounding the textarea, although it does support all browsers that implement non-normative B1 of the 1995 HTML 2.0 specification, which seems to be all of them (including Lynx (tested) and Internet Explorer) (1, 2).