Fix UnexpectedInput.get_context() errors

[jpcano]

I tried to call get_context() like this:

try {
  const result = parser.parse(equation);
  console.log(result.pretty());
} catch (e) {
  console.log("There was an error in the following line");
  console.log(e.get_context(equation));
}

But I got several errors indicating that some symbols are not defined (I execute Lark.js through node v16.18.1)

• max is not defined
• bytes is not defined
• expandtabs is not defined

Correct me if I am wrong but I suspect that this code is autogenerated from Python where those symbols are defined. I rewrote it so it can be executed in my environment. I took the following decisions:

• I assume that text is a string
• I do not try to expand tabs

[erezsh]

Correct me if I am wrong but I suspect that this code is autogenerated from Python

Yes, that's true. And some functions fell through the cracks.

lgtm, thanks!