Fix #7028: Fix lines method of SourcePosition

[odersky]

We now get:

- [E008] Member Not Found Error: i7028.scala:3:5 ------------------------------
2 |  "abc"
3 |    .foo       // error
  |  ^
  |  value foo is not a member of String - did you mean String("abc").+?
-- [E008] Member Not Found Error: i7028.scala:6:5 ------------------------------
5 |  "abc"
6 |    .bar.baz   // error
  |  ^
  |  value bar is not a member of String - did you mean String("abc").+?
-- [E008] Member Not Found Error: i7028.scala:9:5 ------------------------------
8 |  "abc"
9 |    .bar       // error
  |  ^
  |  value bar is not a member of String - did you mean String("abc").+?
three errors found

This looks fine. The current convention is that the ^ points at the start of the range in the first line.
It is hard to combine that with an indicator where the last line ends. So this part is left for future work,
if we decide to reorganize the way positions are presented.