ci: add JSR publishing via github actions

[carafelix]

https://jsr.io/docs/publishing-packages#publishing-from-github-actions

To publish from GitHub Actions, you must first link your package to your GitHub repository from your package settings in JSR. To do this, go to the settings tab of your package on jsr.io, enter your GitHub repository name, and click “Link”.

^^^ Step still needed to do. Haven't created @grammyjs/types since I'm not part of the jsr@grammyjs scope nor have received green light about it

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This workflow will run every time you push to the main branch of your repository. It will publish your package to JSR, and will automatically use the correct version number based on the version in your jsr.json/deno.json(c) file. jsr publish will not attempt to publish if the version specified in your jsr.json/deno.json(c) file is already published to JSR.

[KnorpelSenf]

I still don't understand why this is needed. Can you explain?