Simplify line-column text span string represantation

[KvanTTT]

• [l,c) - if start position equals to the end position.
• [l,c1..c2) - if start line equals to the end line, but columns are different.
• [l1..l2,c) - if start line differs from the end, but columns are the same.
• [l1,c1..l2,c2) - common format.