perf(utils/url): shorten mergePath and optimize for normal use cases.… #6266

Job	Run time
Deno	18s
Checking if it's valid for JSR	10s
Bun	13s
Fastly Compute	52s
AWS Lambda	50s
Lambda@Edge	47s
Main	1m 49s
Node.js v18.18.2	50s
Node.js v20.x	48s
Type & Bundle size Check on Main	1m 2s
Node.js v22.x	51s
Type & Bundle size Check on PR	0s
workerd	50s
Coverage	10s
	9m 30s

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