Var has _var_hooks and _var_imports

Instead of relying on the page/component having
the appropriate imports statically, allow these to
be dynamically calculated for greater flexibility

Ensure that adding additional Var metadata in the
future does not require lots of changes everywhere,
by placing all carryable metadata into _var_data

Remove all* default imports for a page, instead relying
on the components and vars that use a particular
import to name it explicitly with an ImportVar

Remove hardcoded hooks, instead format these in for
components that depend on them.

Move ImportVar to reflex.utils.imports so it can live
next to the ImportDict and avoid circular import with
reflex.vars

It needs to render with curly braces wrapped around it

Instead avoid rendering triggers in Component.render,
but otherwise leave them alone so that components
may be rendered multiple times without changing the
output.

…bstate-rb

Using `rx.cond` helper ensures that the condition carries the `isTrue` import.

Do not surround the formatted values with extra curly braces

…bstate-rb

…bstate-rb

…bstate-rb

Ensure that Vars used in different contexts in a Component are correctly
identified and returned.

…bstate-rb

…bstate-rb

`format_cond` doesn't really work with f-string semantics due to a blind `{`,
`}` removal which breaks the JSON-encoding of VarData. So instead, we carry the
VarData explicitly on the cond_var and change `format_cond` to `str` the Var
before formatting to ensure it does not emit a VarData-encoded f-string.

…bstate-rb

* Do not cast to Style() early in `Component.add_style`
* Memoize return value of `Component._get_vars`
* Defer `VarData.merge` for most operations -- call it once
* Avoid `serializers.serialize` for primitive JSON types