[feature request] select REPL

[rwijtvliet]

From the docs:

The currently supported REPLs are:

[iron.nvim](https://github.com/Vigemus/iron.nvim),
[toggleterm.nvim](https://github.com/akinsho/toggleterm.nvim) or
[molten-nvim](https://github.com/benlubas/molten-nvim)

The latter are automatically detected.

I have both iron and molten installed and would like to try them out (as both seem to have their (dis)advantages). I've noticed, and understand from the quoted text above, that iron is used if installed, even if it's not mentioned in the dependencies table. This does not allow for my use case - I have to remove iron in order to use molten.

IMO it'd be an improvement to check the REPL the user has specified in the dependencies table, and use that one, regardless of which REPLs are additionally installed.