Minor bug: XML files are labeled as "qhelp" #339

Describe the bug

To reproduce
Open an XML file in a workspace that has the CodeQL extension enabled.

Expected behavior

• Query help (.qhelp) files are highlighted as XML
• XML files are still labeled as "XML"

Additional context
@p- recently introduced XML highlighting for Query help files (see #326) ❤️
That update probably got VS Code confused about "normal" XML files.

(Tangential note: I have now added a file association in my user settings, which tells VS Code to treat .qhelp files as XML, without overriding the default XML behavior:

"files.associations": {
    "*.qhelp": "xml"
}

I don't know if it's possible to impose that from within the extension though, so that might not be a helpful approach!)