feat(functions): use isolated-function #600

Job	Run time
matrix	4s
test (@browserless/benchmark)	40s
test (browserless)	1m 26s
test (@browserless/cli)	41s
test (@browserless/devices)	48s
test (@browserless/errors)	47s
test (@browserless/function)	1m 25s
test (@browserless/goto)	1m 46s
test (@browserless/lighthouse)	1m 10s
test (@browserless/pdf)	40s
test (@browserless/screencast)	46s
test (@browserless/screenshot)	1m 2s
test (@browserless/test)	43s
finish	2s
	12m 0s

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