Chore: Move from corepack to devEngines for Node 26

[bachmacintosh]

Description

The Node.js TSC has voted to stop shipping corepack with Node.js beginning with version 25. Since then, version 26 was released earlier this year, and will become the latest LTS version on October 28. This PR updates our mise.toml and package.json files to version both Node.js and pnpm through the devEngines field on package.json. It also removes the .nvmrc file that's been around since before we used mise-en-place to manage the Node.js version, meaning both the runtime and package manager are versioned in one place. This also allows us to use semantic version specifiers for these tools.

Nature of Pull Request

This Pull Request:

• Adds/Updates Type(s)/Schemas
• Adds/Updates Helper Functions/Type Guards
• Updates Documentation
• Updates Project and/or Community Health files (e.g. README, GitHub Actions, etc.)

Its changes constitutes a:

• Non-code change (i.e. no version bump)
• Bug Fix or Other Small Changes (i.e. patch version bump)
• Forward-compatible Enhancement (i.e. minor version bump)
• ⚠️ BREAKING CHANGE regarding TypeScript, Package, and/or WaniKani API version(s) (i.e. MAJOR version bump)

Contribution Terms

I understand that by submitting this Pull Request:

• I agree to the terms outlined in the Contribution Guidelines and Code of Conduct
• My code will be licensed for use in this repository per its LICENSE, and that I have the right to license this code -- per GitHub's Terms of Service

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