feat: use dash notation for inputs (deprecates underscore notation) (#59)

Fixes #57 

This PR implements the 3-step plan proposed by @gr2m in
https://github.com/actions/create-github-app-token/issues/57#issuecomment-1751272252:

> 1. Support both input types
> 2. Log a deprecation warning for the old notation
> 3. Add a test for deprecations

Although this PR supports both input formats simultaneously, I opted
_not_ to document the old format in the updated README. That’s a
decision I’m happy to revisit, if y’all would prefer to have
documentation for both the old and new formats.

lib/post.js

@@ -5,7 +5,9 @@
  * @param {import("@octokit/request").request} request
  */
 export async function post(core, request) {
-  const skipTokenRevoke = Boolean(core.getInput("skip_token_revoke"));
+  const skipTokenRevoke = Boolean(
+    core.getInput("skip-token-revoke") || core.getInput("skip_token_revoke")
+  );
 
   if (skipTokenRevoke) {
     core.info("Token revocation was skipped");