Proposal: Alternate Julia comment syntax

[manuelbb-upb]

Hi,
as always, thanks for the great work!

This is just a proposal for an alternative syntax for julia comments in code blocks.
The usual style is ## comment and this will render as # comment.

However, ## is one of the code cell delimiters in VSCode (which are not user configurable).
Thus, the usual syntax can hinder a code cell oriented workflow.

I implemented #~, added docs and some tests.

• Of course, ~ is up for debate, but it seems to be present on most keyboard layouts and does not clash with comments in code blocks using other languages.
• By the replacement rules I placed in "src/Literate.jl", comments like #~Comment are allowed and render as #Comment.
  The usual ## requires a blank space. What is the rationale behind this?

[manuelbb-upb]

Shortly after I had opened this pull request, there was a change in the vscode extension:
julia-vscode/julia-vscode#2567
Cell delimiters can be configured now.
For example, I disabled the ## delimiter, and in my Literate files I now use #%%#src as my clunky delimiter.

[fredrikekre]

Okay, nice. So this PR isn't needed? Perhaps it can be added to the tips and tricks section in the manual?

[manuelbb-upb]

At least I am not as desperate for this functionality as i was 3 years ago 😁

I still think having a configurable comment pattern in Literate.jl would be cool to have, but realistically I won't find time to update this proposal anytime soon.
Maybe we can convert this to a draft, so as to not forget about it?
But closing is fine with me too.

A note for the vscode configuration in the manual would certainly be nice :)
I'll try to open a separate pull request in the coming days.