feat(experiments): Apply no-code experiments to the webpage.

[Phanatic]

Changes

This PR introduces a new extension to posthog-js called web-experiments which allows posthog to apply no-code experiments to elements on a web page. This PR needs https://github.com/PostHog/posthog/pull/24872 to merge so it can function.

To enable this feature, developers have to opt-in by setting disable_web_experiments to false in posthog.init while in Beta.

There are multiple ways to target an experiment within the browser here :

1. Current URL can be matched with exact, contains, not_contains, regex, not_regex
2. UTM targeting works by parsing the Current URL and evaluating conditions on individual variants.

Checklist

• [x] Tests for new code (see advice on the tests we use)
• [x] Accounted for the impact of any changes across different browsers
• [x] Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

Coming up in following PRs

• Support for dynamic elements on a webpage, via MutationObserver
• Anti-flicker snippet support to prevent flickering of the webpage when applying web experiments.

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Sep 13, 2024 6:59pm

[github-actions[bot]]

Size Change: +19.8 kB (+1.67%)

Total Size: 1.21 MB

Filename	Size	Change
dist/array.full.js	346 kB	+4.96 kB (+1.45%)
dist/array.js	162 kB	+4.95 kB (+3.15%)
dist/main.js	163 kB	+4.95 kB (+3.14%)
dist/module.js	162 kB	+4.95 kB (+3.15%)

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `dist/exception-autocapture.js` | 10.4 kB | | `dist/recorder-v2.js` | 110 kB | | `dist/recorder.js` | 111 kB | | `dist/surveys-preview.js` | 59.8 kB | | `dist/surveys.js` | 66 kB | | `dist/tracing-headers.js` | 8.26 kB | | `dist/web-vitals.js` | 10.3 kB |

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