fix: resolve stale highlights and settings reload issues

[esthor]

This resolves the 2 issues I was encountering, that I think this user was also encountering in this issue: #257

I also wasn't sure if we should keep the comment about the init guarding because it seems more straightforward and safe to just look for the 'enabled' than to check for a negative.

What's Changed:

• Add proper cleanup of decorations when settings change
• Clear all decorations before applying new ones
• Improve handling of extension enable/disable state

[esthor]

btw, I also have an easy repro:

1. go to settings.json
2. create a new custom todohighlight.keywords entry
3. toggle back and forth with the text
4. have a use of that keyword above.
5. watch it not update the styling (on current production version of this extension) BUT watch it update live with the changes in this PR. 🎉

Here's the repro I was using in when running this extension in debug mode:

{
    {
      "// ! This guy will not update unless you have the code changes in this PR active
      "text": "// !", // 👈 change this to anything else and save and see if the above changes immediately or not
      "backgroundColor": "rgba(65, 166, 181, 0.8)",
      "border": "2px solid #41a6b5",
      "isWholeLine": true
    }
}