Implement JSON string escape in pure-awk (several orders of magnitude faster)

[tianon]

I've got a very pathological branch of a certain repository where this makes a parallelized (four concurrent invocations) generation change from ~3 minutes down to ~13 seconds.

I did not implement \b or \f because I don't think the chances of them occurring naturally in a text document (especially a Dockerfile) in 2022+ are very high. 😅

(Also, this isn't required to be pure JSON, but rather "jq compatible" as it's used as jq syntax so we have a little bit more wiggle room as jq is more forgiving than a pure strict parser)

[codecov-commenter]

Codecov Report

Merging #48 (9f6a357) into master (92ab34b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   76.83%   76.83%           
=======================================
  Files           5        5           
  Lines         587      587           
=======================================
  Hits          451      451           
  Misses        104      104           
  Partials       32       32           

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[tianon]

Just running ./apply-templates.sh on PHP takes ~2 minutes currently and ~6 seconds with this change.