chore: add .nvmrc to auto use lts version

[kecrily]

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[turt2live]

Thanks for the PR! Can we get more information in the PR description for what this is aiming to solve and how it helps?

[kecrily]

I was having problems with the node latest version. So I switched to the LTS version.

And if someone is using the node version manager, then its package manager will automatically switch to the latest LTS version after .nvmrc is added. This takes the hassle out of manually switching versions and keeps the development environment as consistent as possible.

[t3chguy]

We don't suggest nvm anywhere, we should instead specify the minimum node version required in the package.json https://docs.npmjs.com/cli/v8/configuring-npm/package-json#engines

[t3chguy]

Ultimately I think this does no harm, thanks for your contribution