feat(remote): add basic --remote support

This is starting from @geekodour's work at
https://github.com/neovim/neovim/pull/8326

runtime/lua/vim/_editor.lua

@@ -636,6 +636,78 @@ function vim.pretty_print(...)
   return ...
 end
 
+local function __rpcrequest(...)
+  return vim.api.nvim_call_function("rpcrequest", {...})
+end
+
+function vim._cs_remote(rcid, args)
+
+  local f_silent = false
+  local f_wait = false
+  local f_tab = false
+  local should_exit = true
+  local command = 'edit '
+
+  local subcmd = string.sub(args[1],10)
+
+  if subcmd == '' then
+    -- no flags to set
+  elseif subcmd == 'tab' then
+    f_tab = true
+  elseif subcmd == 'silent' then
+    f_silent = true
+  elseif subcmd == 'wait' then
+    f_wait = true
+  elseif subcmd == 'wait-silent' then
+    f_wait = true
+    f_silent = true
+  elseif subcmd == 'tab-wait' then
+    f_tab = true
+    f_wait = true
+  elseif subcmd == 'tab-silent' then
+    f_tab = true
+    f_silent = true
+  elseif subcmd == 'tab-wait-silent' then
+    f_tab = true
+    f_wait = true
+    f_silent = true
+  elseif subcmd == 'send' then
+    __rpcrequest(rcid, 'nvim_input', args[2])
+    return { should_exit = should_exit, tabbed = f_tab, files = 0 }
+    -- should we show warning if --server doesn't exist in --send and --expr?
+  elseif subcmd == 'expr' then
+    local res = __rpcrequest(rcid, 'vim_eval', args[2])
+    print(res)
+    return { should_exit = should_exit, tabbed = f_tab, files = 0 }
+  else
+    print('--remote subcommand not found')
+  end
+
+  table.remove(args,1)
+
+  if not f_silent and rcid == 0 then
+    print('Remote server does not exist.')
+  end
+
+  if f_silent and rcid == 0 then
+    print('Remote server does not exist. starting new server')
+    should_exit = false
+  end
+
+  if f_tab then command = 'tabedit ' end
+
+  if rcid ~= 0 then
+    for _, key in ipairs(args) do
+      __rpcrequest(rcid, 'nvim_command', command .. key)
+    end
+  end
+
+  return {
+    should_exit = should_exit,
+    tabbed = f_tab,
+    files = table.getn(args)
+  }
+end
 
 require('vim._meta')