feat: Reactive actions

[adiguba]

Simple implementation for reactive actions on the use:action directive.

• action can be a reactive value, and will be created/destroyed accordingly.
• action can be null/undefined, and will be ignored.

Fix issues : #6754 #6942 #13555

Demo here REPL

Notes :

• This PR changes the signature of the $.action() function. I don't know if this should be considered as an incompatibilit, and if we have to manage the 2 types of calls (before/after this PR)
• I have not created the chanset, because I don't known what king of change to select : patch or minor ?

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]

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Latest commit: e9b2737

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@13953

commit: e9b2737