Do not allow typing into readonly fields.

[jennifer-shehane]

• do not allow typing into readonly form controls as per https://www.w3.org/TR/2010/WD-html5-20101019/common-input-element-attributes.html#attr-input-readonly
• close Validate readonly fields #1246

[brian-mann]

This implementation won't work correctly.

This logic needs to be retry-able - and needs to go within the actionability files, not in cy.type. If you look closely you'll notice that other logic (like disabled inputs) is not in the cy.type implementation for this reason.

The only validation you should do up front are for immutable properties - such as passing invalid arguments to cy.type(...). In those cases, we cannot retry and therefore fail fast and early.

You should review the other cy.type() specs that handle inputs that are hidden, or disabled so you can understand how this should be written.

This PR needs to be reverted until the implementation is fixed - this will otherwise cause spurious failures since the DOM is mutable.