Ensure expected items get nicely printed in error messages

[schneiderfelipe]

In particular, we should review and test when expected items are zero (should assert that never happens), one, two, and three or more things long.

Those lists in errors should follow some rules:

• [ ] lines and column numbers start at 1
• [ ] when expecting a single char

  1:2:
    |
  1 | adc
    |  ^
  unexpected 'd'
  expecting 'b'
  

• [ ] when expecting a single string

  1:1:
    |
  1 | bar
    | ^
  unexpected "bar"
  expecting "foo"
  

• [ ] when expecting a char or a label

  1:3:
    |
  1 | aabbb
    |   ^
  unexpected 'b'
  expecting 'a' or end of input
  

• [ ] when expecting one of many strings

  1:1:
    |
  1 | <empty line>
    | ^
  unexpected end of input
  expecting "data", "file", "ftp", "http", "https", "irc", or "mailto"
  

• [ ] when the cursor is at the end of the input

  1:4:
    |
  1 | irc
    |    ^
  unexpected end of input
  expecting ':'
  

• [ ] when the cursor is at an empty line

  1:1:
    |
  1 | <empty line>
    | ^
  unexpected end of input
  expecting list item
  

Examples above are from Megaparsec (see them here).