Fix parsing format specifiers and refactor (#14685)

* Refactor, extract closures as separate functions

* Add unit test

* Fix the issue with format specifiers

Change parsing format specifiers so that it first checks whether it sees
escaped `%` before it starts parsing flags/width/precision.

* Restore previous behavior but add a warning

* Fix editor test

* Fix hardcoded path in unit test

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Co-authored-by: Adam Boniecki

Author: abonie