Re-prettify errors

[eemeli]

This re-enables the functionality behind the prettyErrors option (default: true). Now in v2 as the information available to errors is a bit different, they also need to get prettified a bit differently.

Errors now point to a specific location in the source, rather than a range. This means that error.linePos contains only what was previously the "start" value.

In order to determine the line/col position, a LineCounter instance may be constructed internally when parsing. If one is provided via the lineCounter option, that'll be used instead.

For errors at the start of a line, the previous line may also be included in the error.message context for it.

> YAML.parse('foo: bar: baz')

YAMLParseError: Nested mappings are not allowed in compact mappings at line 1, column 6:

foo: bar: baz
     ^

    at [...] {
  offset: 5,
  linePos: { line: 1, col: 6 }
}

> YAML.parse('foo: bar\n- baz\n')

YAMLParseError: A block sequence may not be used as an implicit map key at line 2, column 1:

foo: bar
- baz
^

    at [...] {
  offset: 9,
  linePos: { line: 2, col: 1 }
}

> YAML.parse('foo: bar\n- baz\n', { prettyErrors: false })

YAMLParseError: A block sequence may not be used as an implicit map key
    at [...] {
  offset: 9
}