[@xstate/graph][@xstate/test] Add support for `initialState`; add `testModel.getShortestPathsFrom()`

[davidkpiano]

This PR adds support for generating paths from a specific initialState rather than the machine's default initial state.

The use-case for this is for custom path generation, where a dev might want to join paths. For example, if trying to generate a behavioral (given-when-then) test, they would want to:

• Generate path(s) to the given state
• Generate paths starting at the given state and then from the event(s) (the when part)
• Join the first and second set of paths

Another use-case is for E2E test flows where a predefined initial state is not needed; e.g. navigating directly to some page instead of the home page, and running the test model from there.

See here for more info: https://discord.com/channels/795785288994652170/799416911259369472/989132676986437673

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[davidkpiano]

@Andarist @farskid Ping on this review?

[Andarist]

@davidkpiano u have some unaddressed comments from the previous round of review