feature: export TEST_LICENSE_KEY from src/mod.ts for customer use in tests

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[grgbkr]

Can this export go away?

https://github.com/rocicorp/replicache/blob/0b8a99ffb95d63e33d89bd29e95ae9b6348ec8df/src/replicache.ts#L60

Yes, let me remove that one.

[arv]

Did we verify that the JSDoc for this shows up in docs/ ?