Add versioned bundle builder with x-github-api-versions support

This adds a Node.js script to build OpenAPI bundles that include the
API version number using the x-github-api-versions custom extension.

Changes:
- Add scripts/build-versioned-bundles.js for generating versioned bundles
- Update package.json with build scripts and js-yaml dependency
- Document x-github-api-versions extension in extensions.md
- Add output/ to .gitignore

Usage:
  npm run build                    # Build all releases
  npm run build:api.github.com     # Build only api.github.com
  node scripts/build-versioned-bundles.js --versions 2022-11-28

Author: jencarlucci

.gitignore

@@ -1 +1,2 @@
-node_modules/
+node_modules/
+output/

extensions.md

@@ -2,6 +2,30 @@
 
 This document describes the [OpenAPI extensions](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md#specification-extensions) used in GitHub's REST API OpenAPI descriptions.
 
+## `x-github-api-versions`
+
+### Purpose
+
+To specify the API versions supported by this OpenAPI description. GitHub's REST API uses calendar-based versioning (e.g., `2022-11-28`). Clients can use the `X-GitHub-Api-Version` header to request a specific version.
+
+### Usage
+
+This applies to the [Info Object](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md#infoObject).
+
+The value should be an array of strings representing supported API version dates in `YYYY-MM-DD` format, sorted in descending order (newest first).
+
+#### Example usage
+
+```yml
+info:
+  title: GitHub v3 REST API
+  version: 1.1.4
+  x-github-api-versions:
+    - "2022-11-28"
+```
+
+For more information about API versioning, see the [GitHub REST API documentation](https://docs.github.com/rest/about-the-rest-api/api-versions).
+
 ## `x-displayName`
 
 ### Purpose

package.json

@@ -7,13 +7,19 @@
   },
   "license": "MIT",
   "scripts": {
-    "lint": "eslint descriptions/**/*.json --ext .json"
+    "lint": "eslint descriptions/**/*.json --ext .json",
+    "build": "node scripts/build-versioned-bundles.js",
+    "build:versioned": "node scripts/build-versioned-bundles.js --output output/versioned",
+    "build:api.github.com": "node scripts/build-versioned-bundles.js --releases api.github.com",
+    "build:ghec": "node scripts/build-versioned-bundles.js --releases ghec",
+    "build:ghes": "node scripts/build-versioned-bundles.js --releases $(ls descriptions | grep ghes | tr '\\n' ',')"
   },
   "eslintConfig": {
     "extends": ["plugin:json/recommended"]
   },
   "devDependencies": {
     "eslint": "^8.13.0",
-    "eslint-plugin-json": "^3.1.0"
+    "eslint-plugin-json": "^3.1.0",
+    "js-yaml": "^4.1.0"
   }
 }