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/snapshots/index.js.md

@@ -4,6 +4,28 @@ The actual snapshot is saved in `index.js.snap`.
 
 Generated by [AVA](https://avajs.dev).
 
+## action-deprecated-inputs.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `app_id — 'app_id' is deprecated and will be removed in a future version. Use 'app-id' instead.␊
+    private_key — 'private_key' is deprecated and will be removed in a future version. Use 'private-key' instead.␊
+    skip_token_revoke — 'skip_token_revoke' is deprecated and will be removed in a future version. Use 'skip-token-revoke' instead.`
+
+## main-missing-app-id.test.js
+
+> stderr
+
+    'Input required and not supplied: app-id'
+
+> stdout
+
+    ''
+
 ## main-missing-owner.test.js
 
 > stderr
@@ -14,6 +36,16 @@ Generated by [AVA](https://avajs.dev).
 
     ''
 
+## main-missing-private-key.test.js
+
+> stderr
+
+    'Input required and not supplied: private-key'
+
+> stdout
+
+    ''
+
 ## main-missing-repository.test.js
 
 > stderr