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.

package.json

@@ -23,7 +23,8 @@
     "esbuild": "^0.19.4",
     "execa": "^8.0.1",
     "open-cli": "^7.2.0",
-    "undici": "^5.25.2"
+    "undici": "^5.25.2",
+    "yaml": "^2.3.2"
   },
   "release": {
     "branches": [
@@ -48,4 +49,4 @@
       ]
     ]
   }
-}
+}