Mark `next` versions as preview

[spoenemann]

The preview flag in package.json can be set to true to mark a version as a preview version (see extension manifest). I think it would be appropriate to do this for next versions of the built-ins.

[marcdumais-work]

This should already be the case, except for some early builtins we published manually. Anything we build now, old or recent version, should have that field set for "next" versions. Have you seen otherwise?

Looking at a builtin I built locally, I see it at the end of package.json:

"keywords": [
    "Built-in"
  ],
  "repository": {
    "type": "git",
    "url": "https://github.com/theia-ide/vscode-builtin-extensions"
  },
  "preview": true
}

[spoenemann]

Maybe we're just missing to show that flag in the current frontend version, I'll check that.