feat(api): handler.post_load for setting up custom events (#82)

* feat(handler post_load): for setting up custom events
added a post_load callback for handlers to use to set
their own events such as DeferredUIEnter

In addition, made event handler use this feature to create DeferredUIEnter
further decoupling the handlers from the core of lz.n

* Update README.md

Co-authored-by: Marc Jakobi <mrcjkb89@outlook.com>

* Update lua/lz/n/meta.lua

Co-authored-by: Marc Jakobi <mrcjkb89@outlook.com>

---------

Co-authored-by: Marc Jakobi <mrcjkb89@outlook.com>

lua/lz/n/handler/event.lua

@@ -178,4 +178,24 @@ end
 
 M.del = state.del
 
+local deferred_ui_enter = vim.schedule_wrap(function()
+    if vim.v.exiting ~= vim.NIL then
+        return
+    end
+    vim.g.lz_n_did_deferred_ui_enter = true
+    vim.api.nvim_exec_autocmds("User", { pattern = "DeferredUIEnter", modeline = false })
+end)
+
+function M.post_load()
+    if vim.v.vim_did_enter == 1 then
+        deferred_ui_enter()
+    elseif not vim.g.lz_n_did_create_deferred_ui_enter_autocmd then
+        vim.api.nvim_create_autocmd("UIEnter", {
+            once = true,
+            callback = deferred_ui_enter,
+        })
+        vim.g.lz_n_did_create_deferred_ui_enter_autocmd = true
+    end
+end
+
 return M

lua/lz/n/handler/init.lua

@@ -105,6 +105,15 @@ function M.disable(name)
     end)
 end
 
+function M.run_post_load()
+    ---@param handler lz.n.Handler
+    vim.iter(handlers):each(function(_, handler)
+        if handler.post_load then
+            handler.post_load()
+        end
+    end)
+end
+
 ---@param plugins table<string, lz.n.Plugin>
 function M.init(plugins)
     ---@param plugin lz.n.Plugin