[Extensions, IV] Integrate flowR's magic comments

[Ellpeck]

Slicing with flowR now supports magic comments (see
flowr-analysis/flowr#895). They work like this:

Within a document that is to be sliced, you can use magic comments to influence the slicing process:

• # flowr@include_next_line will cause the next line to be included, independent of if it is important for the slice.
• # flowr@include_this_line will cause the current line to be included, independent of if it is important for the slice.
• # flowr@include_start and # flowr@include_end will cause the lines between them to be included, independent of if they are important for the slice. These magic comments can be nested but should appear on a separate line.

They should be highlighted appropriately when using them with the extension. Similar to the TODO Highlight extension (although the type of highlighting should probably be discussed with respect to how prominent it presents itself).