feat(lint): add ignoreIfStatements option to useNullishCoalescing (#10822)

Author: pkallos

.changeset/add-ignore-if-statements.md

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+---
+"@biomejs/biome": patch
+---
+
+Added the option `ignoreIfStatements` to [useNullishCoalescing](https://biomejs.dev/linter/rules/use-nullish-coalescing/). Biome now flags `if` statements that only assign to a nullish variable (such as `if (!a) { a = b }`) and can rewrite them to `??=`. When enabled, Biome ignores those `if` statements.