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 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.
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: 88d8fb5

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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