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/post-token-skipped.test.js

@@ -6,7 +6,7 @@ process.env.STATE_token = "secret123";
 
 // inputs are set as environment variables with the prefix INPUT_
 // https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#example-specifying-inputs
-process.env.INPUT_SKIP_TOKEN_REVOKE = "true";
+process.env["INPUT_SKIP-TOKEN-REVOKE"] = "true";
 
 const mockAgent = new MockAgent();