feat(server): expose function path to context

[dkundel]

This exposes the current function path as the PATH property on the context. This catches up with the
release of that variable in the Twilio Functions environment as introduced in
twilio.com/changelog/twilio-functions-context-path

I marked this as a breaking change because if someone used PATH as an env variable before this will not work anymore.

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[philnash]

When you say breaking change, do you mean you will bump a major version on this? Does the PATH property of context override this if you set it (either locally or in the deployed Functions environment) and has this already changed for all users?