feat: State declarations in class constructors

[elliott-with-the-longest-name-on-github]

There's an annoying problem right now, which comes in about two flavors:

class Counter {
  count = $state<number>();

  constructor(init: number) {
    // oops, forgot to assign init to count, but TypeScript
    // can't help because it thinks it's been assigned above!
  }
}

In this case, TypeScript can't help out because it thinks count has been assigned at its declaration, when it has not!

The other version is:

class PluggableCounter {
  count = $state(0);
  plugin = $state();
  // I shouldn't have to do this
  custom = $derived(this.plugin(this.count));

  constructor(plugin: (c: number) => number) {
    this.plugin = plugin;
  }
}

There are other examples and edge cases ($derived class fields don't play well if you need to reference a non-stateful class field, for example), but the whole class of problem essentially boils down to "sometimes you need to be able to create state in the constructor".

After this PR, you can:

class PluggableCounter {
  count = $state(0);
  custom: number;

  constructor(plugin: (c: number) => number) {
    this.custom = $derived(plugin(this.count));
  }
}

There's a really simple set of rules to follow to declare state in the constructor:

• The field must not already be declared state
• In the constructor, the first assignment to the field must be with the rune
• Creation of the state field can only occur at the top level of the constructor (i.e. not in callbacks or control flow blocks)

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.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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Playground

pnpm add https://pkg.pr.new/svelte@15820

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Preview: https://svelte-dev-git-preview-svelte-15820-svelte.vercel.app/