feat: add isModified helper

[saadkhantareen]

Summary

Adds a public isModified(value) helper, as proposed in #1153. Inside a recipe there is currently no cheap way to ask whether a draft subtree was changed: the available workarounds either snapshot the draft with current() and compare it against original(), or finalize and re-create the draft. isModified answers the question directly from the draft's internal state.

produce(base, draft => {
	performOpaqueUpdates(draft)
	if (isModified(draft.item)) {
		draft.item.modifiedOn = new Date()
	}
})

Changes

• src/utils/common.ts: implement isModified. It returns the draft state's modified_ flag, which markChanged already propagates from a modified nested draft to all of its parent drafts, so modifications anywhere inside the given draft are reported. Values that are not drafts return false.
• src/immer.ts: export isModified from the public entry point.
• src/types/index.js.flow: add the Flow declaration next to isDraft.
• website/docs/api.md: add isModified to the API overview table.
• __tests__/isModified.js: new tests covering non-draft values, unmodified drafts, direct property writes, additions and deletions, same-value assignments (not a modification), nested drafts and untouched siblings, arrays, manual createDraft/finishDraft drafts, Map and Set drafts, and the derived-data use case from isModified helper #1153.

Tests

• npx vitest run __tests__/isModified.js --maxWorkers=2 — 1 file passed, 10 tests passed.
• npx vitest run __tests__/current.js __tests__/original.js __tests__/base.js --maxWorkers=2 — 3 files passed, 3367 tests passed.
• npx tsc -p tsconfig.json — no diagnostics in src/; the only diagnostics reported are inside node_modules/@types/node and are unchanged by this change.

Fixes #1153

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