feat: allow declarations in the template (#18282)

Allows `{let/const ...}` declarations in all places (and more) where we
already allow `{@const ...}` (which will eventually get deprecated in
favor of this new feature).

Closes: #16490


Companion PRs:
- https://github.com/sveltejs/language-tools/pull/3033
- https://github.com/sveltejs/prettier-plugin-svelte/pull/533
- https://github.com/sveltejs/eslint-plugin-svelte/pull/1533
- https://github.com/sveltejs/svelte-eslint-parser/pull/891

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Co-authored-by: Rich Harris <rich.harris@vercel.com>