Move ComputedProperties and CascadedProperties out of the GC. They no
longer contain strong references to GC-managed data.
Keep computed styles alive from DOM elements and animation updates with
RefPtr. Pass style into layout constructors by reference, since layout
only copies the values it needs while building nodes.
Use GC::Weak for cascade source links, so entries no longer keep the
style declaration or shadow root alive.
Previously, the UA stylesheet gave range inputs a definite
`width: 20ch`. This prevented range inputs from filling their grid
track. We now source the default width and height from a new
`RangeInputBox` type. This approach matches the implementation of other
input types.
