LibWeb: Implement deprecated document.execCommand() API

[ADKaster]

The execCommand API is a relic of IE, but is nonetheless implemented by all the major browsers to some extent.

Per the MDN documentation, it has no official specification, only a working draft that was meant to collect the details of the existing implementations for creating new APIs that are more targeted, such as the Clipboard API.

https://developer.mozilla.org/en-US/docs/Web/API/Document/execCommand https://w3c.github.io/editing/docs/execCommand/

There seems to be about 30 'commands' to the execCommand API.

This API does seem to have at least some use in CSS WPT tests.

While it doesn't have a proper spec, it is part of the web platform that we have to support. The HTML editors seem disappointed that there's not a spec for it, perhaps we could write one :P