Ensures observers receive correct initial value

[simonbs]

Description

The changes in this PR ensure that observers installed with spiceStore.$myValue.sink { ... } receive the correct value upon subscribing rather than the default value of the spice.

The value can only be provided when the spice store has been prepared, which happens automatically when reading or writing the value of a spice (e.g., _ = spiceStore.myValue or spiceStore.myValue = newValue), but not when accessing the projected value (spiceStore.$myValue).

As a result, prepareIfNeeded() has been made public, enabling users of Spices to call the function prior to setting up an observer. This is only needed once.

In order to enforce that prepareIfNeeded() is called, projectedValue on a Spice will assert if the underlying storage has not been prepared.

Motivation and Context

Prior to these changes, observers set up with spiceStore.$myValue would receive an incorrect value upon subscribing.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)