This allows users to define behaviors for arbitrary registers. These
registers can be any character including multibyte characters. This
means that any character may be used as a register and if that register
is not a builtin register, it will defer to a user-defined vimscript
function for behavior.

This is done throw an option called 'userregfun'

The function that 'userregfun' defines is a function that takes 3
arguments:

   action - Either set to "put" or "yank"
   register - The character representing the register.
   content - If the action is "yank" this string contains the content
             yanked.

Multibyte registers are still broken for expressions. So while

  let @&=xyz

Works as expected,

  let @λ=xyz

will still throw a parse error.

Problem:    'showbreak' cannot be set for one window.
Solution:   Make 'showbreak' global-local.
https://github.com/vim/vim/commit/ee85702c10495041791f728e977b86005c4496e8

Change in oneleft() is N/A as the relevant condition was removed
(has_mbyte is always true for Nvim, so the condition was always false;
see commit 73dc9e9).

Use wp over curwin for curs_columns().

Required for v8.2.2903 (otherwise test fails as it'll leave the global
option set).

N/A patches for version.c:

vim-patch:8.1.2283: missed on use of p_sbr

Problem:    Missed on use of p_sbr.
Solution:   Add missing p_sbr change.
https://github.com/vim/vim/commit/91e22eb6e09ec384496fccde812072033fd9e616

Already ported in commit 43a874a.

ref #6289

Problem:    Cannot control 'cursorline' highlighting well.
Solution:   Add "screenline". (Christian Brabandt, closes vim/vim#4933)
https://github.com/vim/vim/commit/017ba07fa2cdc578245618717229444fd50c470d

Problem:    'cursorline' always highlights the whole line.
Solution:   Add 'cursorlineopt' to specify what is highlighted.
            (closes vim/vim#4693)
https://github.com/vim/vim/commit/410e98a70bc00ea4bed51e55a8fe20e56a72c087