Improve unknown start of token error message for invisible characters

[CramBL]

Resolves #1016

The approach I implemented is displaying the unknown token in single quotes, but if it's not an ASCII character then it is also escaped and displayed in parenthesis.

Invisible character

Before

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

After

error: Unknown start of token '' (\u{200b}):
  |
1 | test:
  | ^

ASCII character

Before

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

After

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