Print diagnostic messages when debugging mode is enabled

[henrymercer]

This PR prints diagnostic messages to the Actions log when debug logging is enabled by passing debug: true to codeql-action/init or enabling Actions step debug logging.

Example: The following custom query:

ql
/**
 * @kind diagnostic
 * @id test
 */

select "Test diagnostic message", 2

produces:

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.