Support relative negative line ranges

[ajesipow]

Adds support for relative negative line ranges, e.g. -10: or :-10, see #2944.

This is implemented by buffering as many lines as the largest negative offset as a "look ahead" to determine if a line should be printed.

For example:

• for a range -10: 10 lines are buffered and only once EOF is reached will the (last) lines in the buffer be emitted
• for a range :-10 10 lines are buffered and lines are emitted as the buffer moves through the file, once EOF is reached the last lines in the buffer are discarded

[ajesipow]

We may not actually want to add this, since LineRange is pub and a future impl could make it not Copy and therefore require a breaking change.