cmd: Support -s to simplify calls to use New/Errorf

[prashantv]

errtrace.Wrap(errors.New(..)) can be simplified into errtrace.New(...)

Similarly, errtrace.Wrap(fmt.Errorf(...)) can be simplified into errtrace.Errorf(...)

[abhinav]

Note that this will be a slight change in the errtrace transformation concepts:
It's currently modeled as "inserts" that only add at specific positions.
It'll need to change to "edits" that operate over a [start, end) text range it's replacing/modifying.

[abhinav]

With #23, if/when we add -s, it should treat -format=auto as "always"