feat: add workflow to trigger ecosystem-ci from svelte PRs

[dominikg]

see https://github.com/vitejs/vite/blob/main/.github/workflows/ecosystem-ci-trigger.yml for comparison. same thing, just replaced vitejs/vite with sveltejs/svelte

HEADS UP: BIG RESTRUCTURING UNDERWAY

The Svelte repo is currently in the process of heavy restructuring for Svelte 4. After that, work on Svelte 5 will likely change a lot on the compiler aswell. For that reason, please don't open PRs that are large in scope, touch more than a couple of files etc. In other words, bug fixes are fine, but feature PRs will likely not be merged.

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

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

[vercel]

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