Add transpose to the OptionStoreExt

[mahdi739]

This PR adds a method called "transpose" to the OptionStoreExt which converts a subfield with an option into an option of a subfield.
I found it useful in pattern matching while we want to keep the reactivity of the subfield.

[gbj]

Nice! Semver is going to complain here because it adds a new method to the trait, which could in theory be implemented somewhere else. It won't complain if there's a default implementation, though... And I'm pretty sure this can be default-implemented as self.map(|value| value), right?

(In a more general sense, .map() is really just self.transpose().map(map_fn), I guess.)

I guess my one qualm is that transpose() does already exist as a name for something to do with Option<_>, which transposes an Option<Result<T, E>> to Result<Option<T>, E>. On the other hand, this does convey the same sense of "we rearranged the monads!" so maybe it's a fine name after all.

[mahdi739]

Thanks. Actually I wasn't sure about the name so I sent it to get some input. And maybe with the presence of map it's kind of redundant. You can close it or change it or tell me if I should make any change.

[mahdi739]

I made several changes based on your feedback:

• Implemented the new method as a default implementation in the trait to avoid breaking changes.
• Renamed transpose to invert to distinguish it from Rust's similar method.
• Replaced read with try_read in map and map_untracked to handle cases where the signal was disposed, now it returns None in such scenarios.

[gbj]

Awesome. Looks great -- thank you!