Add Writeable WithPart helper (#5328) #2166

Job	Run time
Check Credentials for jobs that need JSON credentials	3s
WASM Demo	2m 2s
bench-binsize	2m 37s
bench-memory (macos-latest)	4m 38s
bench-memory (ubuntu-latest)	3m 50s
bench-memory (windows-latest)	9m 33s
bench-perf (components/datetime)	4m 23s
bench-perf (components/collections)	4m 33s
bench-perf (components/locale)	2m 43s
bench-perf (components/locale_core)	5m 19s
bench-perf (components/segmenter)	3m 44s
bench-perf (components/plurals)	6m 12s
bench-perf (provider/blob)	3m 24s
bench-perf (utils/fixed_decimal)	6m 16s
bench-perf (utils/litemap)	2m 8s
bench-perf (utils/writeable)	3m 41s
bench-perf (utils/tinystr)	6m 39s
bench-perf (utils/zerotrie)	5m 48s
bench-perf (utils/zerovec)	8m 37s
C++ docs	35s
Dart docs	1m 34s
Rust docs	4m 54s
TypeScript docs	39s
Deploy to GitHub Pages	19m 27s
	1h 53m 19s

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