Error get-lexer: 'before-lang-line

#### Error:
```sh 
Caught exn in request "textDocument/documentSymbol"
get-lexer: 'before-lang-line
  context...:
   .../racket-langserver/text-document.rkt:233:0: document-symbol
```

The above error will throw when `#lang ..` is absent or when there are comments ahead of `#lang ..`