Add multiline comment syntax

[developedby]

Is your feature request related to a problem? Please describe.
We'd like to have multiline comment blocks so that we can use them to auto-generate documentation.

It'd allow us to identify what are normal comments and what are documentation comments just syntactically.
It also makes it easier for users to write it without needing to insert a comment in every line.

They can also be used to easily comment out a block of code.

Describe the solution you'd like
"""...""" as a syntax for block comments. This is the syntax python uses.

Block comments can be either a statement or a top-level. They shouldn't be an expression or a term since we might want to reserve it for multiline strings (even though current strings are already multiline, we might want to change that in the future). It would also diverge from python, where """ does not mean a multiline comment in the middle of an expression.

Describe alternatives you've considered

• Other multiline comment separators like ###...###, ##..##, #*...*#, #<...>#, #{...}# etc. I don't particularly like any of them other than ### and we can go with the python behaviour here.
• Not having multiline comments. This is fine and is done in other languages. Doc comments can use an additional marker or use their position regarding the function declaration.