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.

tests/action-deprecated-inputs.test.js

@@ -0,0 +1,16 @@
+import { readFileSync } from "node:fs";
+import * as url from "node:url";
+import YAML from "yaml";
+
+const action = YAML.parse(
+  readFileSync(
+    url.fileURLToPath(new URL("../action.yml", import.meta.url)),
+    "utf8"
+  )
+);
+
+for (const [key, value] of Object.entries(action.inputs)) {
+  if ("deprecationMessage" in value) {
+    console.log(`${key} — ${value.deprecationMessage}`);
+  }
+}