Previously, we only supported very basic numbers and a single level of
text positioning support in the `x`, `y`, `dx` and `dy` attributes in
`<text>` and `<tspan>` SVG elements.
This improves our support for them in the following ways:
* Any `length-percentage` or `number` type value is accepted;
* Nested `<text>` and `<tspan>` use the 'current text position'
concept to determine where the next text run should go;
* We expose the attributes' values through the API.
Though we still do not support:
* Applying the `rotate` attribute;
* Applying transformations on a per-character basis.
* Proper horizontal and vertical glyph advancing (we just use the path
bounding box for now).
The spec defines a generic list interfaces that we can reuse. Currently
we only have SVGTransformList, but we will need this to add
SVGNumberList as well.
Many wpt test on trusted-types relay on this class being defined to even
begin the test as it declares some event handlers.
This is not really an implementation but the most basic setup needed to
run the tests.
