These bindings are generally accesses with <M-Fx> key
presses.

Before it used \x1b<hl>], but this is more consistent with the rest of
Vim.

This allows users to execute commands like:

  let @λ = 'new register content.'

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.

This is done with using embedded tags with the format YourHighlight].

listchars is not the right place for it.

This option allows a character to be displayed in the colorcolumn. This
allows the colorcolumn ot have a traditional line if the colorcolumn
character is set to a vertical line '│' or even a series of dots for a
':'.

The option is a part of the 'listchars' setting under the name
'colorcol'. So 'set listchars=colorcol:│' will work.

(Neo)Vim uses a jump table based on the first character of the option's
name. This means for it to work all options with the same first
character must be together. If this is violated, some options may seem
to disappear, which can sometimes happen during merges if not careful.

This commit adds a check to the gen_options.lua script so the build will
fail if this constraint is violated.