[editing] Add a mechanism to opt-in/opt-out editing features (#93)

There should be a mechanism to allow/disallow a set of editing features.

For example, iPad keyboard has bold, italic, and underline buttons and they should be disabled on a code editor that doesn't support such styling.

However, in the case of contenteditable=true mail client, for example. we may want to opt-out of certain features such as copy/paste instead of opt'ing into support a subset of features.

It seems that we need both an opt-in list as well as an opt-out list as far as we discussed in TPAC.  We may even want to add a new HTMLElement subclass to expose these two lists to avoid polluting the HTMLElement interface.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/editing/issues/93