fix: remove unnecessary `#lang` diagnostic in drracket-created files

[virchau13]

Student language files created by DrRacket do not have #lang directives; they only have #reader directives instead. (See e.g. https://github.com/jeapostrophe/racket-langserver/issues/86). The "Missing #lang" diagnostic should not be reported in these files.

Fixes #86.

[jeapostrophe]

Thank you!