fix: Fix the operation to check for the existence of global var loaded_dial

[gw31415]

Thank you very much for creating such a convenient plugin.

Issue

• plugin/dial.lua is not being loaded.
  • This is the reason why mappings like <Plug>(dial-increment) do not work.

Cause

vim.fn.exists "g:loaded_dial" always returns either 0 or 1.
However, in Lua both 0 and 1 are evaluated as truthy, so the if condition always evaluates to true.
As a result, plugin/dial.lua exits early.

Concern

From a quick search from the issues, it seems this issue has not been reported. Why might that be?

Nevertheless, since I believe this is at least unintended behavior, I have submitted a PR.
Thank you for your consideration.

[monaqa]

Thank you so much! My fault.
There was a bug in #114 which I implemented.
The reason no one reported it is because it was only merged yesterday.