Support Iron.nvim and user functions for sending code to REPL

[mikedecr]

Hi folks, thanks for this package.

When I was trying to set it up for myself, I wanted to hook it into my REPL workflow that uses iron.nvim instead of converting to molten or slime. It seemed easy enough to add an iron.lua runner file, but more generally it seems like the user should be able to pass any fn they want from their config. So this PR does both:

1. Provides cursory support for Iron REPLs plugin.
2. Slightly generalizes the code-sending internals to allow the user to pass any function as their code-runner.

If you don't want to support Iron I would at least encourage you to support functions in the default_method config field. It's nicer than having the user hack into the pkg tables to override the entire send() function.

Thanks, eager to see what you think (fair warning I'm a lua noob and not too familiar with either quarto-nvim or iron code bases).

[jmbuhr]

I really like the idea of passing a function! I'll have a look at this PR some time on the weekend.

[hdemers]

Any plans on merging this PR eventually? I'm very interested in using iron and also the passing of a function that would allow us to play with other similar packages more easily.