Create new rule for image button name descriptive

[EmmaJP]

New PR to replace #1184. This only adds the new rule to check that the accessible name of an image button is descriptive, and does not change any existing rules. As decided in discussions about issue #1049.

Closes issue(s):

• closes and replaces Amendments to button and image button rules #1184
• potentially closes One element relevant for multiple success criteria #1049

Need for Final Call:
This will require a 2 weeks Final Call, as it is a new rule.

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[Jym77]

We are now having a def of attribute value.

Suggested change

	The rule applies to any HTML `input` element with a `type` attribute in the [`Image Button` state](<https://html.spec.whatwg.org/#image-button-state-(type=image)>), that is [included in the accessibility tree][].
	**Note:** `input` elements have a `type` attribute in the `Image button` state if it is set to any case-insensitive match of `image` (most of the time, using `<input type="image">`).
	**Note:** The specification of the [`type`](https://html.spec.whatwg.org/#states-of-the-type-attribute) attribute describes in detail how to map the value of the attribute to its corresponding state.
	The rule applies to any HTML `input` element whose `type` [attribute value][] is `image`, and which is [included in the accessibility tree][].

(+ should also be used in the examples descriptions)

[Jym77]

I think we don't need the assumption here. It is needed to map to 4.1.2 which is concerned about UI components but not to map to 1.1.1 only.

[Jym77]

I think we should keep the same image (and accessible name) for all examples unless a different one is needed. Repetition keeps focus on the important bit: the one that changed.