Add publishing step to OpenVSX

[filiptronicek]

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This PR adds automatic publishing to the OpenVSX registry and therefore fixes #33.

There are three more steps to take cc @chrisjsewell

1. Add a repo secret for GitHub Actions called OPEN_VSX_TOKEN with a PAT obtained via following this guide (steps 1-4)
2. After merging, try publishing for the first time 🚀
3. (Optional) claim the namespace by following this doc so that the extension is verified on the registry.

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[chrisjsewell]

cheers, I'll have a proper look later this week

[filiptronicek]

@chrisjsewell just a friendly ping 🙂, let me know if you have any questions or difficulties.

[ghisvail]

Looking forward to using it on VSCodium.

[ghisvail]

Are there any blockers for this to happen?

[purepani]

Hi! Just wanted to check in on this, as I also use VS codium

[roaldarbol]

Just chiming in, would be super useful and it seems all checks are passing on this PR - let's go OpenVSX. ;-)

[ghisvail]

Is this extension still actively maintained?

Perhaps someone else from the executablebooks community could merge this?

:pray: