Nail down caveats and workarounds when using `accessors: true` (`default-svelte-first` approach)

[vatro]

I'll advise against accessors usage with svelthree-components for svelthree-1.0.0-next.1 until more tests are done and all caveats incl. workarounds are nailed down for the default-svelte-first approach, svelte-accmod aside (for now).

• svelthree tests -> default Svelte using $set vs. default Svelte using accessors:

  • less components + few $set / accessor assignment expressions
  • less components + many $set / accessor assignment expressions
  • many components + few $set / accessor assignment expressions
  • many components + many $set / accessor assignment expressions

• Same as above, but HTML-only (no svelthree usage) tests (for comparison)