Fix hiding of empty boring lines

[benediktwerner]

#1065 introduced a regression where lines in a playpen starting with # but nothing after that are now not hidden anymore. This PR fixes that regression and also adds a test case for this behavior.

[ehuss]

Attributes also seem to be broken. For example:

#![no_std]
#![warn(deprecated_in_future)]
#![warn(missing_docs)]

This is hiding the attributes, and when you expand they are corrupted by removing the #.

[benediktwerner]

Well, as far as I can tell this also wasn't handled properly in the old code and only happened by "accident" because macros are but into a <span> by syntax highlighting and the line then doesn't start with a # anymore but now the lines get hidden before syntax highlighting. I think the correct way has always been to use ##![no_std] which then gets converted to a #![no_std] and stays visible.

But I now changed it so that it doesn't hide lines starting with #! or #[.

[ehuss]

Thank you!!