* Allow `Component.add_style` to return a regular dict
It's more convenient to allow returning a regular dict without having to import
and wrap the value in `rx.style.Style`.
If the dict contains any Var or encoded VarData f-strings, these will be picked
up when the plain dicts are passed to Style.update().
Because Style.update already merges VarData, there is no reason to explicitly
merge it again in this function; this change keeps the merging logic inside the
Style class.
* Test for Style.update with existing Style with _var_data and kwargs
Should retain the _var_data from the original Style instance
* style: Avoid losing VarData in Style.update
If a Style class with _var_data is passed to `Style.update` along with kwargs,
then the _var_data was lost in the double-splat dictionary expansion.
Instead, only apply the kwargs to an existing or new Style instance to retain
_var_data and properly convert values.
* add_style return annotation is Dict[str, Any]
* nit: use lowercase dict in annotation
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