feat: optimize object property mutations in compilation

[caiquetorres]

Currently, when compiling Svelte code containing object property mutations such as object.count++, the Svelte compiler generates verbose JavaScript output involving a closure and unnecessary variable assignment, as demonstrated in the example provided. This PR addresses this inefficiency by optimizing the generated JavaScript output for object property mutations.

During my investigation, I observed that earlier versions of Svelte utilized the mutate function for updating values, which returned the value after modification. Consequently, logic akin to the current compiler-generated output was implemented. However, with the new Svelte version, this output is unnecessary, as simple expressions like object.count++ or object.count += 1 suffice.

The proposed solution involves enhancing the compiler to detect object property mutations and generate streamlined JavaScript output directly corresponding to the intended operation, without the need for additional closures or variable assignments. To achieve this, the compiler now checks the context when converting the code, specifically for the usage of runes.

Closes #10546

Before submitting the PR, please make sure you do the following

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• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 2029219

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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svelte	Patch

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[Rich-Harris]

thank you!