[css-animationworklet] Need a document animator definition and verification

[flackr]

animator definition describes the definition of an animator on the worklet but given that implementations may have multiple worklets we need a document animator definition concept (analogous to document paint definition) and document animator definition map which describes the main context's concept of valid animators. This should be referred to when creating AnimationWorklets, and definitions verified or rejected when registering animators (Similar to step 22 of Registering a custom paint).

[majido]

This should be fixed now.

The PR adds document animator definition and corresponding map. The map is consulted and updated during registration process.At the moment, the only possible difference between two registration is the "statefullness". In future if we add inputProperties those could also create potential for difference.

Also updated the prose to use map from infra spec which is moreconcrete and in line with paint worklet spec.