[Feature Request]: Persistent `:BufferLinePick` letters

### What?

In buffer-pick mode, currently the letters for each buffer are set on-the-fly, so they will not persist between sessions, meaning that you have to re-learn the letters if there are two filenames that start with the same letter, but you open their buffers in different orders in two separate sessions.

It would be nice if this plugin could remember the letters between sessions. We could invent the concept of "buffer sets" which are associated with the current working directory. Whenever we open a file for the first time in some directory, we assign a letter to it and save this to disk (under this directory's buffer set). If this file has already been assigned a letter in another buffer set, we use that same letter _unless_ it conflicts with one of the letters in the current buffer set, in which case it is re-assigned in all the buffer sets (this is the only case when a letter can be re-assigned) so that there is no conflict with any of its buffer sets.

### Why?

This feature should quickly allow you to pick buffers under the current project without having to look at the label (that is, as soon as you have learned them), and not have to worry about them changing when you quit and re-open vim. We can also think about allowing this to work for buffers that have not yet been opened (in which case bufferline will open them for you).