Use --json-format for gcov reports to get pre-structured results

[MichaelChirico]

h/t @aitap for highlighting this:

#604 (comment)

See docs:

https://gcc.gnu.org/onlinedocs/gcc/Invoking-Gcov.html

And thankfully we already import {jsonlite}:

covr/DESCRIPTION

Line 51 in d8d33b4

jsonlite,

---

Currently, gcov output is parsed with regular expressions:

covr/R/compiled.R

Lines 18 to 37 in d8d33b4

	re <- rex::rex(any_spaces,
	capture(name = "coverage", some_of(digit, "-", "#", "=")),
	":", any_spaces,
	capture(name = "line", digits),
	":"
	)
	matches <- rex::re_matches(lines, re)
	# Exclude lines with no match to the pattern
	lines <- lines[!is.na(matches$coverage)]
	matches <- na.omit(matches)
	# gcov lines which have no coverage
	matches$coverage[matches$coverage == "#####"] <- 0 # nolint
	# gcov lines which have parse error, so make untracked
	matches$coverage[matches$coverage == "====="] <- "-"
	coverage_lines <- matches$line != "0" & matches$coverage != "-"

I think it will be much more robust and extensible to use the structured JSON output instead.