Add Debug button to #lang htdp/asl.

[mikesperber]

Fixes #177.

[mikesperber]

@rfindler This avoids the "two syntax-check buttons" problem.

[mikesperber]

This also makes https://github.com/racket/drracket/pull/592 obsolete.

[racket-discourse-github-bot]

This pull request has been mentioned on Racket Discourse. There might be relevant details there:

https://racket.discourse.group/t/racket-v8-16-draft-release-notes/3518/2