src/vs/workbench/contrib/extensions/browser/extensionsWorkbenchService.ts/Extension/getChangelog
class Extension:	async getChangelog(token: CancellationToken): Promise<string> {
		const local = this.getLocal();
		if (local?.changelogUrl) {
			const content = await this.fileService.readFile(local.changelogUrl);
			return content.value.toString();
		}

		if (this.gallery?.assets.changelog) {
			return this.galleryService.getChangelog(this.gallery, token);
		}

		if (this.type === ExtensionType.System) {
			return Promise.resolve('Please check the [VS Code Release Notes](command:update.showCurrentReleaseNotes) for changes to the built-in extensions.');
		}

		return Promise.reject(new Error('not available'));
	}