LibWeb: Ensure properties using discrete interpolation are non-transitionable

[tcl3]

If a property is uses discrete interpolation and TransitionBehavior is not set to AllowDiscrete that property should be non-transitionable.

This is now true for properties whose animation type is not discrete, but the animation type falls back to discrete.

This gets us +1055/-426 WPT subtest passes on various interpolation tests.

[tcl3]

My latest push makes interpolate_value() return an empty optional rather than returning interpolate_discrete() everywhere.