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-lock.json

@@ -20,7 +20,8 @@
         "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"
       }
     },
     "node_modules/@actions/core": {
@@ -4286,6 +4287,15 @@
       "resolved": "https://registry.npmjs.org/yallist/-/yallist-4.0.0.tgz",
       "integrity": "sha512-3wdGidZyq5PB084XLES5TpOSRA3wjXAlIWMhum2kRcv/41Sn2emQ0dycQW4uZXLejwKvg6EsvbdlVL+FYEct7A=="
     },
+    "node_modules/yaml": {
+      "version": "2.3.2",
+      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.3.2.tgz",
+      "integrity": "sha512-N/lyzTPaJasoDmfV7YTrYCI0G/3ivm/9wdG0aHuheKowWQwGTsK0Eoiw6utmzAnI6pkJa0DUVygvp3spqqEKXg==",
+      "dev": true,
+      "engines": {
+        "node": ">= 14"
+      }
+    },
     "node_modules/yargs": {
       "version": "17.7.2",
       "resolved": "https://registry.npmjs.org/yargs/-/yargs-17.7.2.tgz",