Use `outputVar` instead of hard coding `this.output` in JS codegen #1557

Job	Run time
compiler (ubuntu-20.04)	2m 39s
compiler (ubuntu-22.04)	2m 7s
lib-java-kotlin	5m 29s
lib-rust	3m 21s
lib-go (1.21)	1m 51s
lib-php (7.1)	49s
lib-php (7.2)	54s
lib-go (1.22)	2m 17s
lib-swift	52s
lib-python (3.x)	1m 23s
lib-php (7.3)	49s
lib-php (7.4)	51s
lib-php (8)	45s
lib-php (8.1)	51s
lib-php (8.2)	1m 1s
lib-php (8.3)	1m 4s
cross-test (java, java,kotlin)	1m 39s
cross-test (java, go,rs)	2m 0s
cross-test (java, swift)	37s
cross-test (kotlin, java,kotlin)	36s
cross-test (kotlin, go,rs)	49s
cross-test (kotlin, swift)	52s
cross-test (go, java,kotlin)	1m 15s
cross-test (go, go,rs)	1m 16s
cross-test (go, swift)	29s
cross-test (rs, java,kotlin)	44s
cross-test (rs, go,rs)	1m 9s
cross-test (rs, swift)	50s
cross-test (swift, java,kotlin)	55s
cross-test (swift, go,rs)	1m 26s
cross-test (swift, swift)	26s
	42m 6s

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