Add GPX to XML extensions

[nickswalker]

GPS Exchange Format is a common XML schema. The root XML tag is left out often enough that it warrants adding to linguist.

Description

Adds .gpx to XML extensions and a rootless sample of my creation for testing.

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • 6.5k .gpx without <?xml: https://github.com/search?q=NOT+is%3Afork+path%3A*.gpx+NOT+%22%3C%3Fxml%22&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s): Self-created
    • Sample license(s): Self-created
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Important

The changes in this PR will not appear on GitHub until the next release has been made and deployed. See here for more details.