Support julia markdown mode

[gdkrmr]

Weave.jl supports .jl files by prefixing the markdown parts wit #' [1], similar to R documentation [2]. It would be cool to have the following features:

• When in a line that starts with #', inserting a line break should prefix the new line automatically it with #'.
• M-q should be able to reflow text correctly (it currently keeps the ' when reflowing).
• Also when typing #' I get a second ' autocompleted.

[1] https://weavejl.mpastell.com/stable/publish/ and https://weavejl.mpastell.com/stable/examples/FIR_design_plots.jl

[2] https://roxygen2.r-lib.org/