[REF-889] useContext per substate

[masenf]

This PR lays the groundwork for independently operating substates in their own contexts. Why would we want this? Because components that only depend on a substate do not need to re-render when some unrelated part of the state changes.

Fix REF-889 Separate context per child state

The state dict() and get_delta() functions now both return data in the same format:

{
    state.get_full_name(): all_vars,
    ...
}

This flat structure is easier to work with on both ends, and now the wire format of both full dict and partial delta look the same and can be used interchangably.

In contexts.js.jinja2, the code now iterates through the initialState and creates a separate context with reducer+dispatch for each top-level key.

In state.js when an update comes in, look up the associated dispatcher and apply the subdelta to it. applyDelta becomes a dead simple spread operation now that each substate is just a flat dictionary.

Fix REF-1128 Var should have imports and hooks

Also Fix REF-564 Var operations should carry state.

Now each Var has a _var_data field which stores information about what hooks and imports are needed in any component that includes that Var. When rendering a component, there is a new _get_vars iterator that yields all vars used by the component. Any hooks or imports collected from these are added to the components hooks/imports.

This has allowed the removal of nearly all default imports in reflex/compiler/compiler.py, since the Vars that use particular imports now carry that information themselves.

With the addition of _var_data comes _replace method, which is used internally by Var operations to make a copy of the Var with some attributes overwritten. This helps preserve state and var_data across operations.

Because _var_data.imports and _var_data.hooks are collections, operations involving multiple vars will accumulate imports and hooks, allowing for arbitrary combination of vars without losing this dependency information.

This structure will also enable more advanced Var use cases, such as wrapping component-local useState hooks for client-side only component state.

Other Changes

ImportVar was moved into reflex.utils.imports to reduce annoyances around circular imports with constants and var modules.

[masenf]

@Lendemor @picklelo @ElijahAhianyo this one is fixed up and ready to the best of my knowledge

[picklelo]

Code works great for me, but left some comments on code style.

I think for any non-trivial function we need to consistently block the code into chunks separated by newlines, where each chunk has a full sentence comment. Not necessarily breaking out into sub functions, but keeping individual function code clean by segmenting it whenever we have conditionals, loops, or pieces of logic, and clearly commenting each of these segments with a comment. Then it can also be easy to validate of the code is doing what the comment says. As we add more complex logic, it will help keep the code more readable and maintainable.

[picklelo]

Shouldn't style go in here also?

[masenf]

self.style itself will never be a Var, but it might have Var values inside of it, which is why it's handled specially.

[masenf]

I merged main back into this branch after the compile optimization PR merged and these changes regress compile time by 3 seconds total. Will need to poke around with the profiler and see if i can speed up these changes.

Edit: was able to shrink the regression down to only ~1.5 - 2s on reflex-web