Positions in the Language Server Protocol and Build Server Protocol are
line/column-based instead of offset-based, so this is more convenient.
Computing the line/column from the offset is possible but requires
reading the source file.
A position now has a start, an end, and a point (the existing `offset`),
just like it does in the Scala compiler. This information is especially
useful for displaying squiggly lines in an IDE.
This commit and the next one are required for https://github.com/sbt/zinc/pull/571