ci: configure dependabot updates for GitHub Actions #323

Job	Run time
check	2m 37s
test-firefox	1m 45s
test-chrome	1m 43s
test-firefox-webworker	1m 46s
test-node (windows-latest, lts/*)	3m 38s
test-node (ubuntu-latest, lts/*)	1m 29s
test-node (macos-latest, lts/*)	3m 24s
test-webkit (ubuntu-latest, lts/*)	36s
test-webkit (macos-latest, lts/*)	3m 25s
test-webkit-webworker (ubuntu-latest, lts/*)	40s
test-webkit-webworker (macos-latest, lts/*)	3m 17s
test-electron-renderer	47s
test-electron-main	1m 40s
test-chrome-webworker	1m 29s
release	0s
	28m 16s

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