fix: make bind_this implementation more robust

[dummdidumm]

The previous bind_this implementation had flaws around timing and didn't work correctly when the binding wasn't $state in runes mode. This PR reimplements bind_this by aligning it more with how Svelte 4 bindings work, namely reacting to each block context variables updates properly and introducing a mechanism to get the previous binding destination using said variables.

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

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• 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: 9a488d5

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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