- always set `Document.type` and `Document.contentType`
- `Document.createElement` properly HTML casing and (X)HTML namespacing

https://dom.spec.whatwg.org/#dom-domimplementation-createhtmldocument
https://dom.spec.whatwg.org/#dom-document-createelement

when `mimeType` is `text/html`.
The `mimeType` can now optionally be passed to the `DOMHandler` constructor.
Documented `DOMHandler` constructor and all properties.

- For XML documents the XHTML and SVG mime types are preserved as expected.
- `Document.documentURI` is no longer initialized with the undocumented `Locator.systemId` value.
- Deprecate `DOMParserOptions.domBuilder` since state would be preserved between calls to `DOMParser.parseFromString` which can have unexpected side effects, especially since we are now using the `DOMHandler` to manage the mimeType and defaultNamespace.

to be able to copy from options provided to `DOMParser`

Instead of accessing `this.options` in `DOMParser.parseToString`,
the default values are now applied in the constructor.
Since the locator passed to `options` is no longer being modified,
the type of the option was changed to boolean.
There is no change in behavior in this commit,
since truthy and falsy values are accepted as well.

Instead of accessing `this.options` in `DOMParser.parseToString`,
the default values are now applied in the constructor.

Instead of accessing `this.options` in `DOMParser.parseToString`,
use `this.errorHandler`.

which points to a class instead of an instance and is only meant for internal testing.

BREAKING CHANGE: If you used to configure `DOMParserOptions.domBuilder`.
You might be able to configure the `domHandler` instead, but should be avoided.
This is only there for testing purposes.

All options are now taken care of by the constructor and are available as individual properties.
Most are marked as `readonly`, some are `private`.

BREAKING CHANGE: If you used `DOMParser.options` after creating an instance.
You can still read the individual properties from the instance,
but there is no longer a way to mutate them, so you need to really pass the required options when constructing them.

in HTML docs or namespaces

https://html.spec.whatwg.org/#boolean-attributes
https://html.spec.whatwg.org/#boolean-attributes
https://html.spec.whatwg.org/#attributes-3

and drop warning for boolean attributes in HTML

BREAKING CHANGE: The following methods no longer allow a (non spec compliant) boolean argument to toggle "HTML rules":
- `XMLSerializer.serializeToString`
- `Node.toString`
- `Document.toString`