[js] Certain extended classes in web API lack appropriately mutable fields

For example, `DOMMatrix` which extends `DOMMatrixReadOnly` should make properties representing matrix components writable. In Haxe's js target these properties remain immutable:

~~~haxe
import js.html.DOMMatrix;
var matrix = new DOMMatrix();
matrix.a = 1;
~~~

Compiler says: `This expression cannot be accessed for writing`

See also `DOMPoint` and `DOMRect`.

Start by locating the Haxe js-target declarations for DOMMatrix, DOMMatrixReadOnly, DOMPoint, and DOMRect, then compare how inherited fields are declared. Use the DOMMatrix example as a compile check and confirm that the relevant component properties are writable while the read-only base remains unchanged.

Written by the indexing model from the issue text.