[feat] add Action as a type for use:action

[ivanhofer]

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• This message body should clearly illustrate what problems it solves.
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Tests

• Run the tests with npm test and lint the project with npm run lint

Currently there is no type definition for a Svelte action defined. This PR adds a new Action type that get's exposed by the svelte package.

This can be used to type actions that are declared outside of a Svelte component.

action.ts

import type { Action } from 'svelte'

export const myAction: Action<HTMLTextAreaElement, boolean> (node, param) => {
   // `node` and `param` are typed 
   return {
      update: (param) => void 0
      destroy: () => void 0
   }
}

export const myOtherAction: Action<HTMLTextAreaElement> (node) => {
   return {
      destroy: () => void 0
   }
}

The benefit of this type is that you will get an error if you define something wrong. e.g. you misspelled 'update' as 'updates'.

The Svelte VS Code extension already throws an error if you try to apply the action use:myAction, but only if you have opened that file that uses the action. You won't see an error if you only open the action.ts file in your IDE.

[dummdidumm]

See #7121, this already exists to some extent. The "void extends" clause is interesting for differentiation though, maybe we can incorporate that.

[ivanhofer]

uups, forgot to rebase so I haven't seen this type in my forked repo😅

[tanhauhau]

Close in favor of #7442

i've incorporated your idea for "void extends" into #7121, and attributed you as co-author. thanks!

[ivanhofer]

@tanhauhau thanks, I completely forgot about this