Footprint improvement for browser

[guybedford]

Improves the last PR to only add the unhandled rejection handler in the Node.js build.

[github-actions]

File size impact

Merging node-opt into master will impact 3 files in 2 groups.

browser (2/2)

File	raw	gzip	brotli	Event
dist/s.min.js	-24 (7,471)	-7 (2,939)	-6 (2,664)	modified
dist/system.min.js	-24 (11,614)	-7 (4,451)	-10 (4,008)	modified
Total size impact	-48 (19,085)	-14 (7,390)	-16 (6,672)

node (1/1)

File	raw	gzip	brotli	Event
dist/system-node.cjs	+37 (201,419)	+6 (52,156)	+42 (43,839)	modified
Total size impact	+37 (201,419)	+6 (52,156)	+42 (43,839)

extras (0/8)

No impact on files in extras group.

_{Generated by file size impact during size-impact#424476572}