Add `isolated()` wrapper style.

[oscbyspro]

Description

This style was proposed in #108 for manual ownership of a style's cache.

Details

• Its own cache should be of type Void.
• It should store the wrapped style’s cache and inject it into the style’s methods.
• It should store the wrapped style's cache as reference shared among all copies of an instances.
• It should store the wrapped style using value semantics.

[oscbyspro]

Implemented in 2469618 (untested). I will add tests before v5.0.0.