.xhtml and .xht files are being validated as HTML instead of XHTML

[geekley]

For example, using <br> instead of <br/> should generate an error in .xhtml files, which use XML parser.
https://validator.w3.org/nu/#file does properly recognize the XHTML extensions and use the appropriate parser.
https://validator.github.io/validator/ also claims to do the same.

It seems all capabilities exist in Nu Html Checker, the extension just needs to detect it and use the functionality (vscode doesn't use a different language id for XHTML).

XHTML is expected when:

• file is server as mime type application/xhtml+xml or application/xml; or
• filename ends with .xht or .xhtml (when mime type is unknown, e.g. local files)