Provide basic XML syntax highlighting for qhelp files #325

Problem
As a developer it would be nice to have basic syntax highlighting support for qhelp files.
Currently qhelp files are displayed as plain text and look rather dull ;)
Also there's no help for finding out which tags belong together.

Proposed solution
Since qhelp files are basically XML files interpreting them as XML files would be a low hanging fruit.
This would not automatically XML validation support as VS code does not support XML validation out of the box.

Describe alternatives you've considered
An alternative would be to use a different VSCode grammar set that restricts the set of tags that can be used. However, I think that would be a better candidate for XML validation.

Additional context
I will create a small pull request for this issue.
The Pull request instructions instructed me that issues need to be created for any UI or other user-facing changes, so I created this one.