fix(core): unstable_promise option in write getter

[dai-shi]

This experimental feature is first introduced in #539.
We don't get much feedback, and it's still experimental.
But, we hope to make it stable in the future. For now, this makes it an option object and no longer warns.

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[github-actions]

Size Change: +23 B (0%)

Total Size: 40.7 kB

Filename	Size	Change
dist/esm/index.js	4.3 kB	+8 B (0%)
dist/index.js	5.17 kB	+15 B (0%)

ℹ️ View Unchanged

Filename	Size
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