editor-injector: add extensions gallery to product.json in start server script

MR: feat: add extensions gallery to product.json fo... (!70 - merged)

Description

Add the following key in the product.json

"extensionsGallery": {
    "serviceUrl": "https://open-vsx.org/vscode/gallery",
    "itemUrl": "https://open-vsx.org/vscode/item",
    "resourceUrlTemplate": "https://open-vsx.org/api/{publisher}/{name}/{version}/file/{path}"
}

https://github.com/eclipse/openvsx/wiki/Using-Open-VSX-in-VS-Code talks about serviceUrl and itemUrl but does not mention resourceUrlTemplate without which it does not work. resourceUrlTemplate is used by the feature that allows exploring themes outside the extension marketplace using the command palette in VS Code.