feat(border): add border to main window (#80)

This adds support for window border

Features:
* Add window border on vertical side of the window

Preview:
![Screenshot
(58)](https://user-images.githubusercontent.com/38243998/214766890-43155a64-3a96-42d6-ac54-3672601afbde.png)

lua/zen-mode/config.lua

@@ -3,6 +3,7 @@ local M = {}
 
 ---@class ZenOptions
 local defaults = {
+  border = "none",
   zindex = 40, -- zindex of the zen window. Should be less than 50, which is the float default
   window = {
     backdrop = 0.95, -- shade the backdrop of the zen window. Set to 1 to keep the same as Normal

lua/zen-mode/view.lua

@@ -178,6 +178,7 @@ function M.create(opts)
   local win_opts = vim.tbl_extend("keep", {
     relative = "editor",
     zindex = opts.zindex,
+    border = opts.border,
   }, M.layout(opts))
 
   local buf = vim.api.nvim_get_current_buf()
@@ -219,7 +220,7 @@ function M.fix_hl(win, normal)
     vim.api.nvim_set_current_win(win)
   end
   normal = normal or "Normal"
-  vim.cmd("setlocal winhl=NormalFloat:" .. normal)
+  vim.cmd("setlocal winhl=NormalFloat:" .. normal .. ",FloatBorder:ZenBorder")
   vim.cmd("setlocal winblend=0")
   vim.cmd([[setlocal fcs=eob:\ ,fold:\ ,vert:\]])
   -- vim.api.nvim_win_set_option(win, "winhighlight", "NormalFloat:" .. normal)