feat: better support for reloading with lazy.nvim

## Details

The goal is being able to run `:Lazy reload render-markdown.nvim` in
order to quickly check changes to configuration as well as code.

After some recent changes `extmarks` can sometimes get stuck forever as
the cache gets reloaded but the buffer is never fully cleared.

To improve this 2 minor changes were made:

- use `vim.api.nvim_list_bufs()` when clearing the namespace instead of
  the keys in `M.cache`, that way we capture previously cached buffers
- in addition to hiding all the `extmarks` when clearing, add a call to
  `vim.api.nvim_buf_clear_namespace` to make sure no longer visible
  marks are also removed

Author: MeanderingProgrammer

doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*        For NVIM v0.11.2        Last change: 2025 June 21
+*render-markdown.txt*        For NVIM v0.11.2        Last change: 2025 June 22
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

lua/render-markdown/core/ui.lua

@@ -27,7 +27,7 @@ M.cache = {}
 function M.setup(config)
     M.config = config
     -- clear marks and reset cache
-    for buf in pairs(M.cache) do
+    for _, buf in ipairs(vim.api.nvim_list_bufs()) do
         vim.api.nvim_buf_clear_namespace(buf, M.ns, 0, -1)
     end
     M.cache = {}
@@ -133,6 +133,7 @@ function Updater:clear()
     for _, extmark in ipairs(extmarks) do
         extmark:hide(M.ns, self.buf)
     end
+    vim.api.nvim_buf_clear_namespace(self.buf, M.ns, 0, -1)
     M.config.on.clear({ buf = self.buf, win = self.win })
 end
 

lua/render-markdown/health.lua

@@ -5,7 +5,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '8.5.9'
+M.version = '8.5.10'
 
 function M.check()
     M.start('version')