feat: serverlist({peer=true}) returns peer addresses #34806

Problem:
serverlist() only lists servers that were started by the current Nvim.

Solution:
Look for other Nvim servers in stdpath("run").

runtime/lua/vim/_core/server.lua

@@ -0,0 +1,33 @@
+local M = {}
+
+--- Called by builtin serverlist(). Returns all running servers.
+--- in stdpath("run"). Does not include named pipes or TCP servers.
+---
+--- @param listed string[] Already listed servers
+--- @return string[] A list of currently running servers in stdpath("run")
+function M.serverlist(listed)
+  -- TODO: also get named pipes on Windows
+  local socket_paths = vim.fs.find(function(name, _)
+    return name:match('nvim.*')
+  end, { path = vim.fn.stdpath('run'), type = 'socket', limit = math.huge })
+
+  local running_sockets = {}
+  for _, socket in ipairs(socket_paths) do
+    -- Don't list servers twice
+    if not vim.list_contains(listed, 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(running_sockets, socket)
+        end
+        vim.fn.chanclose(chan)
+      end
+    end
+  end
+
+  return running_sockets
+end
+
+return M

runtime/lua/vim/_meta/vimfn.lua

@@ -7964,12 +7964,20 @@ function vim.fn.searchpos(pattern, flags, stopline, timeout, skip) end
 
 --- Returns a list of server addresses, or empty if all servers
 --- were stopped. |serverstart()| |serverstop()|
+---
+--- The optional argument {opts} is a Dict and supports the following items:
+---
+---   peer  : If |TRUE|, servers not started by |serverstart()|
+---           will also be returned. (default: |FALSE|)
+---           Not supported on Windows yet.
+---
 --- Example: >vim
 ---   echo serverlist()
 --- <
 ---
+--- @param opts? table
 --- @return string[]
-function vim.fn.serverlist() end
+function vim.fn.serverlist(opts) end
 
 --- Opens a socket or named pipe at {address} and listens for
 --- |RPC| messages. Clients can send |API| commands to the