Syntax checker off-by-one in error reporting

[justinnoah]

When building a project (monte build), the ^~~~~ is off by one to the left.

E.g.:

  67     ""
  68     def consumer :Consumer := makeConsumer(key, secret)
  69     def token := null
            ^~~~~~
  70 
  71     return object Client:
  72         to request(uri :Str, meth, body :Bytes, headers :Map):
Unused name token