Add a sprintf-like version of --output

[petdance]

We currently use eval for implementing --output. This is fraught with peril.

Redo --output without eval, and maybe with a sprintf-like formatting system.

[n1vux]

xref Ack2 discussion under beyondgrep/ack2#517 and https://github.com/petdance/ack2/issues?q=is%3Aopen+label%3A--output

[n1vux]

My additional desire is ability to create KWIC/KWOC index which would be much nicer with sprintf in --output.
(Yes, i'd need a post sort but shell is good at piping to sort.)

for better KWIC/KWOC alignment
ack2 --output can sorta do KWOC/KWIC with evil before/after vars
--output '$&^I$'"'"'^I|| $' # KWOC --output '$^I$&^I$'"'" # pseudo KWIC
but they’re nasty from Shell since must mix quote and dollar
and tabs don’t truly line up if width variation exceeds a tab width
need a sprintf with field-width for real thing …
(Our current ack2 --output "evalmanages to not eval'sprintf("%20s\t%20s\t%20s",$,$&,$'"'"')' which is good for security but foils my KWIC/KWOC plans. Unless I use the eval evil i used to break eval previously.)

Related: should allow and recommend use English; vars be used in --output
(is it already? No. In Ack2, $& works but $MATCH does not. Nor does ${^MATCH} )

Related: need to have a variables available to the sprintf that are Filename and $. relative to filename.