Fix whitespace

Integrate with `<form>` elements directly through built-in support for
[ElementInternals][].

This is achieved with graceful degradation in two ways:

* automatically check for browser support
* support globally disabling through `Trix.config.editor.formAssociated = false`

According to the [Form-associated custom elements][] section of [More
capable form controls][], various behaviors that the `<trix-editor>`
element was recreating are provided out of the box.

For example, the `<label>` element support can be achieved through
[ElementInternals.labels][]. Similarly, a `formResetCallback()` will
fire whenever the associated `<form>` element resets.

Add support for integrating with [Constraint validation][] through the
support for the `[required]` attribute and the
`setCustomValidity(message)` method.

[Constraint validation]: https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation
[#1023]: #1023
[ElementInternals]: https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals
[Form-associated custom elements]: https://web.dev/articles/more-capable-form-controls#form-associated_custom_elements
[More capable form controls]: https://web.dev/articles/more-capable-form-controls
[ElementInternals.setFormValue]: https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals/setFormValue
[ElementInternals.labels]: https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals/labels

…e-for-csp

Read the csp meta tag nonce attribute and fall back to content

Delegate implementation-specific methods and properties to the
appropriate delegate. Set the delegate based on the
`TrixEditorElement.formAssociated` property.

Support for `ElementInternals` and Constraint Validations

Resolve a [CI failure][] related to a hard-coded English validation
message expectation failing due to an internationalized actual
validation message value.

[CI failure]: https://github.com/basecamp/trix/actions/runs/11317691021/job/31471435810#step:5:75

Fix CI: `.validationMessage` assertion