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?