feat(prefer-const): add rule

[mikededo]

Adds a new rule named prefer-const which:

• Overrides the ESLint `prefer-const rule to support Svelte syntax, as discussed
  in New Rules: rune-prefer-let & prefer-const #818.
• Ports Proxies the original prefer-const rule from ESLint, adapting it for Svelte.

This PR is an updated version of #816, which did not include types. Also, some
opinionated changes were made in terms of code style, not in terms of behavior.
Avoided re-implemeting the core rule, by using the getCoreRule and proxying the
default implementation.

I found that this rule could be very useful for Svelte, and there could be many
developers who would benefit from it. At the same time, I thought that the
initial implementation was a good starting point, and that adding types and
would make the rule more robust to changes.
However, the rule should not have many changes, as how variables are handled in
JS is not likely to change.

Acknowledgement: this rule is based on the prefer-const rule from ESLint and
the initial work done by @bfanger.

[changeset-bot]

🦋 Changeset detected

Latest commit: df7abaa

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
eslint-plugin-svelte	Minor

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[mikededo]

Since this rule logic has been adapted from the original prefer-const, I plan on adding the same tests as there are in the prefer-const implementation (see the test source).

I feel it's mandatory to also evaluate all the expected behaviour, not only what happens in *.svelte.

[ota-meshi]

Thanks for working on implementing this rule!
However, could you please implement it using getCoreRule() in the same way as no-inner-declarations?
https://github.com/sveltejs/eslint-plugin-svelte/blob/main/packages/eslint-plugin-svelte/src/rules/no-inner-declarations.ts

I think it just avoids calling listeners on VariableDeclaration that use runes.

I imagine it would probably be something like this:

	create(context) {
		return defineWrapperListener(coreRule, context, {
			createListenerProxy(coreListener) {
				return {
					...coreListener,
					VariableDeclaration(node) {
						for (const decl of node.declarations) {
							if (
								decl.init?.type === 'CallExpression' &&
								decl.init.callee.type === 'Identifier' &&
								['$state', '$props', '$derived'].includes(decl.init.callee.name)
							) {
								return;
							}
						}
						coreListener.VariableDeclaration?.(node);
					}
				};
			}
		});
	}

[mikededo]

Thanks @ota-meshi for pointing this out, my initial idea was to use a proxy for the rule, to avoid having a complete re-write. Should've asked before spending so many hours 😅 🤦🏼

Will look into it. Maybe I'll end up closing this PR in favour of another one.

[ota-meshi]

Thank you for this PR!