feat(vimfn): use Lua for more excmds/vimfns

Problem:
Too much boilerplate needed to use Lua to impl an excmd or f_xx
function.

Solution:
- Add `nlua_call_vimfn` which takes the args typval, executes
  Lua, and returns a typval.
- refactor(excmd): lua impl for :log, :lsp

runtime/lua/vim/_core/server.lua

@@ -2,36 +2,40 @@
 
 local M = {}
 
---- Called by builtin serverlist(). Returns all running servers in stdpath("run").
+--- Called by builtin serverlist(). Returns the combined server list (own + peers).
 ---
---- - TODO: track TCP servers, somehow.
---- - TODO: support Windows named pipes.
----
---- @param listed string[] Already listed servers
---- @return string[] # List of servers found on the current machine in stdpath("run").
-function M.serverlist(listed)
+--- @param opts? table Options:
+---              - opts.peer is true, also discover peer servers.
+--- @param addrs string[] Internal ("own") addresses, from `server_address_list`.
+--- @return string[] # Combined list of servers (own + peers).
+function M.serverlist(opts, addrs)
+  if type(opts) ~= 'table' or not opts.peer then
+    return addrs
+  end
+
+  -- Discover peer servers in stdpath("run").
+  -- TODO: track TCP servers, somehow.
+  -- TODO: support Windows named pipes.
   local root = vim.fs.normalize(vim.fn.stdpath('run') .. '/..')
   local socket_paths = vim.fs.find(function(name, _)
     return name:match('nvim.*')
   end, { path = root, type = 'socket', limit = math.huge })
 
-  local found = {} ---@type string[]
   for _, socket in ipairs(socket_paths) do
-    -- Don't list servers twice
-    if not vim.list_contains(listed, socket) then
+    if not vim.list_contains(addrs, socket) then
       local ok, chan = pcall(vim.fn.sockconnect, 'pipe', socket, { rpc = true })
       if ok and chan then
         -- Check that the server is responding
         -- TODO: do we need a timeout or error handling here?
         if vim.fn.rpcrequest(chan, 'nvim_get_chan_info', 0).id then
-          table.insert(found, socket)
+          table.insert(addrs, socket)
         end
         vim.fn.chanclose(chan)
       end
     end
   end
 
-  return found
+  return addrs
 end
 
 return M