build/lib/extensions.ts/isWebExtension
function isWebExtension(manifest: IExtensionManifest): boolean {
	if (Boolean(manifest.browser)) {
		return true;
	}
	if (Boolean(manifest.main)) {
		return false;
	}
	// neither browser nor main
	if (typeof manifest.extensionKind !== 'undefined') {
		const extensionKind = Array.isArray(manifest.extensionKind) ? manifest.extensionKind : [manifest.extensionKind];
		if (extensionKind.indexOf('web') >= 0) {
			return true;
		}
	}
	if (typeof manifest.contributes !== 'undefined') {
		for (const id of ['debuggers', 'terminal', 'typescriptServerPlugins']) {
			if (manifest.contributes.hasOwnProperty(id)) {
				return false;
			}
		}
	}
	return true;
}