feat(experiments): Introduce new Funnels stats methods

[danielbachhuber]

See #26713

Changes

Introduces a new set of methods for calculating Funnels values:

• calculate_probabilities_v2
• are_results_significant_v2
• calculate_expected_loss_v2
• calculate_credible_intervals_v2

Uses the new methods when stats_version=2 is present in the results request.

v2 stats

v1 stats

How did you test this code?

Tests should pass. I also compared an existing funnel experiment with its new results.

[github-actions]

Size Change: 0 B

Total Size: 1.11 MB

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frontend/dist/toolbar.js	1.11 MB

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[andehen]

The stats looks good! I've simulated a bunch of experiments, and all the results made sense.

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