fix: refactor ls to use output.buffer for json

[lukekarrys]

This has no functional difference but matches where
we landed for the rest of the commands.

[npm-cli-bot]

no statistically significant performance changes detected

timing results

app-large	clean	lock-only	cache-only	modules-only	no-lock	no-cache	no-modules	no-clean	show-version	run-script	cache-only peer-deps	no-clean audit
npm@latest	27.647 ±0.53	10.954 ±0.04	11.930 ±0.08	1.571 ±0.01	1.541 ±0.02	1.299 ±0.02	8.417 ±0.13	1.294 ±0.00	0.141 ±0.00	0.168 ±0.00	14.981 ±0.23	2.141 ±0.07
#7542	30.659 ±4.03	10.827 ±0.07	11.863 ±0.04	1.579 ±0.04	1.604 ±0.04	1.313 ±0.03	8.322 ±0.02	1.319 ±0.02	0.143 ±0.00	0.171 ±0.00	14.905 ±0.06	2.162 ±0.10

app-medium	clean	lock-only	cache-only	modules-only	no-lock	no-cache	no-modules	no-clean	show-version	run-script	cache-only peer-deps	no-clean audit
npm@latest	21.602 ±0.45	8.014 ±0.03	8.942 ±0.04	1.522 ±0.02	1.499 ±0.02	1.408 ±0.00	5.870 ±0.01	1.326 ±0.01	0.140 ±0.00	0.168 ±0.00	9.979 ±0.01	1.996 ±0.06
#7542	22.103 ±0.20	8.077 ±0.03	8.976 ±0.02	1.543 ±0.02	1.506 ±0.01	1.400 ±0.00	5.871 ±0.04	1.322 ±0.02	0.140 ±0.00	0.167 ±0.00	9.876 ±0.06	2.033 ±0.01