Formalize $(...) as operator for reactive store binding

[iacore]

Describe the problem

Currently, only top lever reactive store variables can be used with the syntax $var_name. $(list[0].text), $let_binding are all illegal.

By making $(any, expression) valid, in addition to $identifier would solve the problems of #3617.

Describe the proposed solution

https://svelte.dev/repl/ca5c3ef8604a44ffa096a792d5d0fa86?version=3.44.3

Alternatives considered

The same functionality can be achieve by making each code fragment a new component. This would leave us with components with a few lines and no meaning outside the only parent component it is used.

Importance

would make my life easier

[Conduitry]

Duplicate of #4079.