feat(options): lua closure/function options

Problem:
Cannot assign Lua functions/closures to "func" ('completefunc',
'tagfun', …) or "expr" ('foldexpr', 'indentexpr', …) options.

Solution:
- Store "func"/"expr" options as `Callback` instead of string.
- Delete oceans of copy-pasted code.
- BREAKING: LuaRef returned via RPC/Vimscript is now represented as
  `"<Lua N: file:line>"` (like what `:map` shows) instead of `nil`.
- Note: `man.vim` still uses `v:lua` string, bc it's a vimscript ftplugin.

Helped-by: Lewis Russell <lewis6991@gmail.com>

runtime/lua/vim/lsp.lua

@@ -846,12 +846,12 @@ function lsp._set_defaults(client, bufnr)
   if
     client:supports_method('textDocument/definition') and is_empty_or_default(bufnr, 'tagfunc')
   then
-    vim.bo[bufnr].tagfunc = 'v:lua.vim.lsp.tagfunc'
+    vim.bo[bufnr].tagfunc = lsp.tagfunc
   end
   if
     client:supports_method('textDocument/completion') and is_empty_or_default(bufnr, 'omnifunc')
   then
-    vim.bo[bufnr].omnifunc = 'v:lua.vim.lsp.omnifunc'
+    vim.bo[bufnr].omnifunc = lsp.omnifunc
   end
   if
     client:supports_method('textDocument/rangeFormatting')