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.

.github/workflows/test.yml

@@ -38,8 +38,8 @@ jobs:
       - uses: ./ # Uses the action in the root directory
         id: test
         with:
-          app_id: ${{ vars.TEST_APP_ID }}
-          private_key: ${{ secrets.TEST_APP_PRIVATE_KEY }}
+          app-id: ${{ vars.TEST_APP_ID }}
+          private-key: ${{ secrets.TEST_APP_PRIVATE_KEY }}
       - uses: octokit/request-action@v2.x
         id: get-repository
         env: