Undeprecate Position.toSingleLine

[scabug]

The toSingleLine method in Position was deprecated in 1282395572b699ca85d4c1dbf3b16f7dad3e9732 and given focus as replacement. However, that is not at all the same thing:

• the original method returned another range position, cut to the end of the first line
• focus returns an offset position (length is 0)

I suggest the method is undeprecated.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8780?orig=1
Reporter: @dragos
Affected Versions: 2.11.2