Use objects for validation errors

[schlessera]

Feature description

(Copied from #3780 (comment))

We should use objects for the different validation errors (like you would use separate objects for exceptions as well).

Benefits:

1. Instead of passing arrays around with multiple types of information, we can pass a single object of type AMP_Validation_Error around.
2. The objects can have proper constructors that know which type of data to collect for each error, allowing us to add type checking and other safeguards:

   return new DisallowedFirstChildError( $child_elements[0]->nodeName, $child_tags['first_child_tag_name_oneof'] );`

3. If we have variations for a given error, we can have named constructors instead of the built-in one. (Don't think that is needed, though)
4. All the display message generation can be encapsulated:

   foreach ( $validation_errors as $error ) {
   	$title = $error->get_title();
   	// [...]
   }

5. Faster, as it replaces string comparisons with type checks:

   // if ( $validation_error['code'] === AMP_Tag_And_Attribute_Sanitizer::DISALLOWED_ATTR ) {}
   if ( $validation_error instanceof DisallowedAttributeError ) {}

6. Uses less memory, because most arrays with strings will be turned into internal properties instead.
7. Removes big switch() statements like the one below. Whatever needs to be checked across all validation errors will be encapsulated as a method on them.
8. I personally think it is much easier to read (see 5. as an example).

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Will be important as part of #3730.