Unable to put unicode characters in #attributes

Problem/Motivation

#attributes gets converted into a \Drupal\Core\Template\Attribute object, with each attribute either needing to extend AttributeValueBase or being converted to one of Attribute[Array|Boolean|String]. These all call Html::escape on their values when converting to a string.

It is common to use data attributes (e.g. data-icon) to store icon characters coupled with CSS (e.g. content: attr(data-icon);) to output icons. It's common practice to use unicode html entities for this (e.g. ✔ for a tick). These then get escaped and render garbage.

Proposed resolution

Add a AttributeEscaped that you can use to indicate that you have already appropriately filtered and escaped the attribute value, who's __toString simply returns the value.

API changes

Addition of a new attribute for use in contrib.