Allow completers to choose partial completions #971
Description
Right now, with partial completions enabled, Reedline will automatically complete the common prefix of the completer's suggestions. But some completers might be using super-smart algorithms that Reedline doesn't understand, or they might need to add quotes around the partial completions.
So, completers should have the option to also return an optional partial completion with its generated suggestions. It would be a string and a span, and if the user hits TAB or whatever, the given span would be replaced with the given string.
References
None right now, I think Zsh allows this though.