Add warning for use of session store outside of a component

[ghostdevv]

Describe the problem

You are unable to use the session store outside of a component and currently you just get an error message along the lines of Function called outside component initialisation

Describe the proposed solution

Talked in the discord about potentially adding something to svelte core to allow you to check whether you are in component initialisation or not and then libs can use that to throw more helpful error messages

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[rmunn]

This is probably moot now that #5946 has removed the special $session store. (It can now be implemented quite easily by anyone who needs it, using event.locals and a +layout.server.ts file). So I'm closing the issue; if the decision to remove the session store gets reversed, we can reopen the issue if/when that happens.