fix: check if highlighter is active before computing concealed regions

## Details

Issue: #300

While we report inactive highlights in the healthcheck we may as well
handle the case when highlights are not active accurately. To do this
all we need to do is check if highlights are active for the buffer
before iterating through the highlighter query.

Author: MeanderingProgrammer

Diff for: doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*          For 0.10.0         Last change: 2025 January 15
+*render-markdown.txt*          For 0.10.0         Last change: 2025 January 16
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

Diff for: lua/render-markdown/core/conceal.lua

@@ -86,6 +86,9 @@ function Conceal:compute(context)
     if not self:enabled() then
         return
     end
+    if vim.treesitter.highlighter.active[self.buf] == nil then
+        return
+    end
     local parser = vim.treesitter.get_parser(self.buf)
     context:parse(parser)
     parser:for_each_tree(function(tree, language_tree)

Diff for: lua/render-markdown/health.lua

@@ -5,7 +5,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '7.8.8'
+M.version = '7.8.9'
 
 function M.check()
     M.start('version')