computed var default to cache=True

[Lendemor]

Set the default value of cache to True for @rx.var

Lead to better performance "out of the box" since any computed var will be cached, unless explicitly disabled.

Potentially breaking some apps behaviour, so we are considering delaying this PR to 0.7.0.

[masenf]

can we actually make the signature cache: bool = None

then in ComputedVar.__init__, we can check if cache is None and throw a deprecation warning, keeping the default as False until 0.7.0; then we remove the warning and replace the None default with True.

If users want to get rid of the warning, then they will need an explicit value for the cache parameter.

The main downside with this approach is that @rx.var decoration (with no parentheses) will raise the deprecation warning until 0.7.0 is released... maybe that's not ideal

[masenf]

marking as draft to avoid accidentally merging a behavior-changing PR