Refactor: use a single .json file to describe all cached artifact bundles (#121)

This is a pure refactor, moving from a separate .cache file per bundle to a single cache-metadata.json file describing all bundles. Instead of storing cache metadata in a separate .cache file per artifact bundle, all of the metadata is now stored in a single `.json` file.

This will make it easier to implement more flexible artifact-caching strategies, such as caching each wrapper zip separately.

* Always include cache protocol version in cache key
* Store all cache metadata in a single JSON file
* Rename cache-metadata file and bump protocol version
* Polish and documentation

src/post.ts

@@ -6,7 +6,9 @@ import * as caches from './caches'
 // throw an uncaught exception.  Instead of failing this action, just warn.
 process.on('uncaughtException', e => handleFailure(e))
 
-// Invoked by GitHub Actions
+/**
+ * The post-execution entry point for the action, called by Github Actions after completing all steps for the Job.
+ */
 export async function run(): Promise<void> {
     try {
         await caches.save()