Improve unknown start of token error message for invisible characters

[casey]

This came up because someone had a BOM in their justfile, which just was choking on.

The error message looked like:

error: Unknown start of token:
  |
1 | test:
  | ^

We should print the unknown start of token message such that invisible characters show up, for example by surrounding the offending character in quotes. We could also think about displaying non-printing characters as unicode escape sequences. Perhaps there's a function in the standard library that helps with that.