R.nvim, take 2

[daler]

R.nvim

This adds support for R.nvim.

This:

• is not required for using R (you can always use toggleterm if you want)
• adds some nice new features, but
• needs additional steps to use (Ropen; nvim and R must be running on the same host)

See the new docs for more.

attach-ipython

This PR also adds a new function, attach-ipython, which lets you run IPython to debug a conda env...without needing to install IPython into that env.

minor

And last, the dictionary source for the blink tab-completion plugin will only be added if the dictionary file exists. Sometimes an interactive node will not have that file, which previously would result in repeated errors but this is now fixed by enabling that source only if the file exists.