feat(core): add memoization to style generation

[CaerusKaru]

Fixes #887

[CaerusKaru]

NOTE: A follow-up will be needed to cache the values for builders that run adjacent to other tasks

[CaerusKaru]

Ok so the follow-up was a little hasty. The layout issue was resolved plainly and easily by adding a sideEffect parameter to addStyles. Basically it allows you to get the computed styles and perform an operation with them after the styles have been added (the logic being that if you need to change style generation itself, it should be done in the builder).

For gap, the story is different because it depends a lot on the directionality of the given moment. So, while caching is done in grid mode, it's not really possible at any other point. Although this is not ideal, it's an unfortunate reality.

[CaerusKaru]

Conceptually this raises issues with fxLayoutGap because it doesn't actually leave much room for the StyleBuilder to be independent. A closer look will have to be taken at that particular directive.

[CaerusKaru]

Ok fixed that too because now the sideEffects are part of the StyleBuilders. That way there's a clear-cut path for the chain of events: StyleBuilder.buildStyles -> StyleBuilder.sideEffect. This way the buildStyles values can be cached if necessary, and the sideEffect computation can happen regardless.

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