Add xmllinter #211

Job	Run time
Quality control	36s
Security checks	17s
Linter	1m 22s
format('Unit tests, PHP {0}, {1}', matrix.php-versions, matrix.operating-system)	0s
format('Unit tests, PHP {0}, {1}', matrix.php-versions, matrix.operating-system)	0s
Code coverage	0s
Cleanup artifacts	-1s
	2m 14s

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