Using auto_suggest for copilot-like completions

I'd like to use auto_suggest for copilot like AI auto-completions. However, there seem to be some complications with doing this:

• auto_suggest only supports completing at the end of the prompt.
• auto_suggest doesn't really seem to work with multiline suggestions. Any newlines in the suggestions are printed as ^J. Additionally, the transformers seem to work line-by-line, so it's not so clear how to even write something that could suggest multiple lines.
• The AppendAutoSuggestions processor is hard-coded in the input processors https://github.com/prompt-toolkit/python-prompt-toolkit/blob/669541123c9a72da1fda662cbd0a18ffe9e6d113/src/prompt_toolkit/widgets/base.py#L240. Meaning even if I write my own processor to handle the above issues, I can't really use AutoSuggest as that processor would try to handle any suggestion that is put on the buffer.

Start by reading auto_suggest, AppendAutoSuggestions, and the input-processor setup in src/prompt_toolkit/widgets/base.py. Trace how suggestions are limited to the prompt end and how multiline text is transformed and rendered. Done would require a clear design for copilot-like, multiline or custom suggestion handling, including how the hard-coded processor should interact with it.