Migrate from Coveralls to Codecov for coverage reporting #92

Job	Run time
PHPUnit (8.2, lowest)	23s
Psalm	21s
PHPUnit (8.3, highest)	18s
PHPUnit (8.4, highest)	19s
PHPUnit (8.2, highest)	20s
Coding Standard / Coding Standard (PHP 8.2)	13s
PHPUnit (8.5, highest)	20s
	2m 14s

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