fix: disallow exporting `actions` from a `+layout.server` file

[teemingc]

~Found out we can export actions in a layout file but it isn't mentioned anywhere in the docs.~ I don't think we're suppose to export actions from a layout file. https://github.com/sveltejs/kit/pull/10046#issuecomment-1565286897

This PR changes the list of valid exports to disallow the actions export from a +layout.server file (matching the language tools behaviour).

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[david-plugge]

I don´t think you can do that. Vscode gives me this error:

Invalid export 'actions' (valid exports are prerender, ssr, csr, trailingSlash, config, load, or anything with a '_' prefix

[teemingc]

Huh. That's weird. It's suppose to be valid based on the code here: https://github.com/sveltejs/kit/blob/f8cf1fec360961724fb00d3b47513a4058cfd411/packages/kit/src/utils/exports.js#L72 and it's tested here https://github.com/sveltejs/kit/blob/f8cf1fec360961724fb00d3b47513a4058cfd411/packages/kit/src/utils/exports.spec.js#L114-L118

[teemingc]

Not sure if we need to update language tools to allow actions in layout or update kit to disallow actions in layout. https://github.com/sveltejs/language-tools/blob/98b925617bbdbf6e9e51a1d7e229d0f27c2337cc/packages/typescript-plugin/src/language-service/sveltekit.ts#L280-L281

[david-plugge]

I wonder how actions in layout files would work, specifically how you would call them from the client (what to put in the form action attribute?). Page and layout actions could collide, how would the server choose between src/routes/#layout.server.ts#actions.signin and src/routes/#page.server.ts#actions.signin ?

[elliott-with-the-longest-name-on-github]

I... would expect you shouldn't export actions from layout components? I have no idea what URL they would / should live at.