chore: auto publish alpha versions

[pauldambra]

Sometimes we want to test changes without releasing them generally Let's allow ourselves to publish alphas more easily

This trigger type only fires if the workfllow file is on the default branch so I can't test it exclusively in one PR

This PR introduces a workflow file that runs when a specified label is added

The workflow

• checks the person has write access to posthog-js
• pushes an alpha prerelease to NPM

To Follow-up

it also isn't clever enough to e.g. push alpha-1, alpha-2 etc so if someone ran it twice on one version of posthog-js then it would fail but my goal today is to push an alpha of https://github.com/PostHog/posthog-js/pull/1411 so I can figure out how to use it and then test it

to follow-up to this PR would then be detecting a sensible alpha version to publish

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Sep 13, 2024 8:49am

[github-actions[bot]]

Size Change: 0 B

Total Size: 1.19 MB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `dist/array.full.js` | 341 kB | | `dist/array.js` | 157 kB | | `dist/exception-autocapture.js` | 10.4 kB | | `dist/main.js` | 158 kB | | `dist/module.js` | 157 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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