test(svelte-5): add dedicated tests

[metonym]

Closes #2267

This library is intended to work with Svelte 3, 4, and 5 (non-runes mode). Currently, this library uses Svelte 4 to test and type-check the source code. It also uses Svelte 4 for the interactive demos in the docs.

This PR adds scaffolding to run the same testing with the latest Svelte 5 version.

Motivation

• Basic coverage, supporting Compatibility with Svelte 5 #2012
• Avoid regressions like [Svelte 5] Tabs causes effect_update_depth_exceeded error #2366 caused by fix(tabs): resolve index de-sync from dynamic tab rendering #2350
• Identify incongruencies betwee 4/5, and fix as necessary

Changes

• Add a minimal scaffold in tests-svelte5. This must be a separate package since mixing Svelte versions in the same package.json has not worked well (causes trouble for svelte-check, Vite plugin etc..)
• Migrate existing tests to use the new Svelte Testing Library API (rerender instead of component.$set)
• Flag Svelte 5 quirks (implying bugs that need addressing)

if (isSvelte5) {
  // Svelte 5 may emit check event multiple times
  expect(consoleLog.mock.calls.length).toBeGreaterThanOrEqual(2);
} else {
  expect(consoleLog).toHaveBeenCalledTimes(2);
}