CI: add publishing to Open VSX Registry. Closes #315.

[alek13]

Before merge:

• follow the first 2 steps of Official Publising in Open VSX Registry #315
• generate PAT here and save at secrets.OVSX_ACCESS_TOKEN of repo

Can be done after merge:

• follow the 4th step of Official Publising in Open VSX Registry #315

[alek13]

@CppCXY , can you merge this ? Or still have questions ?
FYI: I think you don't need to have a license file when publishing. For example this one: https://open-vsx.org/extension/theo/emmylua published with link right to this repo... (not sure, but we can try).

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EDIT: license is needed, but it is not necessary to have a file, you can simply specify it in package.json in field { "license": "MIT" }