Set up automated publication for Open VSX

[yhatt]

Currently Marp for VS Code is availbale in Open VSX Registry: https://open-vsx.org/extension/marp-team/marp-vscode (3652928)

Microsoft is banning by terms of use to use the extension, downloaded from VS marketplace, in the other IDE. By publishing extension into Open VSX, we can provide alternative IDE users easier way to install the extension without extra worries.

Open VSX is adopted by VSCodium and Eclipse Theia. In future, code-server also will adopt it as a primary marketplace. (https://github.com/cdr/code-server/blob/main/docs/FAQ.md#how-can-i-request-a-missing-extension)

I've published .vsix manually, but it's better to set up automated publication based on current vsix that has uploaded to GitHub releases, for reducing cost of maintainance.

Automated publication is mainly targeted VSCodium. Please notice Theia has not yet supported VS Code extension procotol for extending Markdown preview. Related: #28