Add `iferr` integration (#15)

* feat(iferr): add to installer & config

* feat(iferr): add first implementation

* docs(iferr): add

README.md

···
 
 8. Generate doc comment
 
-First set a cursor on **public** package/function/interface/struct and execure:
+First set a cursor on **public** package/function/interface/struct and execute:
 
 ```vim
 :GoCmt
 ```
 
-## Thanks:
+9. Generate `if err`
+
+Set cursor on the line with **err** and execute:
+
+```vim
+:GoIfErr
+```
+
+## Thanks
 
 - [go.nvim](https://github.com/ray-x/go.nvim)

lua/gopher/config.lua

···
       gomodifytags = "gomodifytags",
       gotests = "gotests",
       impl = "impl",
+      iferr = "iferr",
     },
   },
 }

lua/gopher/iferr.lua

···
+local c = require("gopher.config").config.commands
+local u = require "gopher._utils"
+
+---Add iferr declaration
+---That's Lua of vimscript implementation of:
+---github.com/koron/iferr
+return function()
+  local boff = vim.fn.wordcount().cursor_bytes
+  local cmd = (c.iferr .. " -pos " .. boff)
+  local data = vim.fn.systemlist(cmd, vim.fn.bufnr "%")
+
+  if vim.v.shell_error ~= 0 then
+    u.notify("command " .. cmd .. " exited with code " .. vim.v.shell_error, "error")
+    return
+  end
+
+  local pos = vim.fn.getcurpos()[2]
+  vim.fn.append(pos, data)
+  vim.cmd [[silent normal! j=2j]]
+  vim.fn.setpos(".", pos)
+end

lua/gopher/init.lua

···
 gopher.get = require "gopher.goget"
 gopher.impl = require "gopher.impl"
 gopher.generate = require "gopher.gogenerate"
+gopher.iferr = require "gopher.iferr"
 gopher.comment = require "gopher.comment"
 gopher.test_add = gotests.func_test
 gopher.test_exported = gotests.all_exported_tests

lua/gopher/installer.lua

···
   gomodifytags = "github.com/fatih/gomodifytags",
   impl = "github.com/josharian/impl",
   gotests = "github.com/cweill/gotests/...",
+  iferr = "github.com/koron/iferr",
 }
 
 ---@param pkg string

plugin/gopher.vim

···
 command! -nargs=* GoImpl :lua require"gopher".impl(<f-args>)
 command! -nargs=* GoGenerate :lua require"gopher".generate(<f-args>)
 command! GoCmt :lua require"gopher".comment()
+command! GoIfErr :lua require"gopher".iferr()
 command! GoInstallDeps :lua require"gopher".install_deps()