feat: add Marko example and include code coverage for Marko files #6355

Job	Run time
lint	2m 20s
test-browser-safari	7m 49s
test-browser (chrome, chromium)	5m 16s
test-browser (firefox, firefox)	5m 41s
test (ubuntu-latest, 16)	14m 8s
test-browser-windows (chrome, chromium)	9m 6s
test-browser (edge, webkit)	5m 22s
test (ubuntu-latest, 18)	19m 46s
test-ui	2m 4s
test-browser-windows (edge, webkit)	9m 21s
test (macos-latest, 18)	17m 25s
test (windows-latest, 18)	9m 46s
typecheck	4m 27s
	1h 52m 31s

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