Previously if a highlight group with a name outside the regexp
[a-zA-Z0-9_] was defined, Nvim would emit an "invalid character"
warning message. This was annoying for Lua scripts, as it was very hard
to debug what line of code was triggering this message since it didn't
produce a stack trace.

This has now been promoted to an error with the code E5248.

Additionally the ASCII character period ('.') and at-sign ('@') have
been added to the allowed list of characters of a highlight group name
to support the application of defining hierarchical highlight groups,
e.g. 'TS.keyword'.

Co-authored-by: Christian Clason <christian.clason@uni-due.de>

This change unlocks additional registers for Neovim by allowing a user
to define their own behavior for non-builtin registers.

This is accopmlished through a new option 'userregfunc'

The 'userregfunc' defines the function to call when handling a register
for which there is no builtin functionality.

The 'userregfunc' function should take 3 arguments:

   action - Either "yank" or "put"
   register - The character corresponding to the register
   content - In the case of action == "yank", the dictionary describing
             the yanked content, with the following keys:

             {type} - Either "char", "line" or "block"
             {lines} - The lines being yanked as a list
             {width} - The width in case of "block" mode.
             {additional_data} - Additional data (can be returned in
             "put" mode)

   In case of "put" this function should return the content to put. This
   content can be either:

     * A dictionary in the same template as content above.
     * A list of strings. This will be assumed to be "line" mode.
     * A string. This will be assumed to be "char" mode.

An example of a "null" 'userregfunc' that provides an implementation
identical to traditional vim registers would be:

      let s:contents = {}
      function! MyUserregFunction(action, register, content) abort
         if a:action == "put"
           return get(s:contents, a:register, "")
        else
           let s:contents[a:register] = a:content
        endif
      endfunction
      set userregfun=MyUserregFunction

  It is important to note that any valid unicode character can now be a
  register, including something like @☺.

  This change also addresses the multibyte parsing issues surrounding

  let @a = 'xyz'
  let @🔨 = 'hammer'

Update runtime files
https://github.com/vim/vim/commit/63f32603789d1a27c559fc440325955fd0b8b500

skip translations
skip user manual rewrite

Update runtime files.
https://github.com/vim/vim/commit/016188fd8a30cfbaca3faa0daea9a47138dc5c4b

omit changes from doc/builtin.txt (needs 8.2.1536, 8.2.4981)
skip user manual rewrite for Vim9script

Problem:    Checking translations affects the search pattern history.
Solution:   Use "keeppatterns". (Doug Kearns)
https://github.com/vim/vim/commit/8a3704723c40779d688ef957dbe5bd8b65c25f95