Adds a plain custom string input component to the CLI plugin init

[brob]

Description

This adds a new "starter" plugin to the sanity init plugin command.

This is in the hopes that a clear, clean custom input component plugin can help folks get started a bit quicker at creating their own.

What to review

It'll be good to test the CLI for it, as well as take a look at the plugin it will install which you can find here: https://github.com/sanity-io/sanity-plugin-custom-string

Notes for release

sanity init plugin now has an option to create a starting point for a custom string input to help create your next custom input plugin.

[vercel[bot]]

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test-studio – ./

🔍 Inspect: https://vercel.com/sanity-io/test-studio/Bg3ssgPoekwpAEQM64fNNeGFmhmw
✅ Preview: https://test-studio-git-plain-custom-input-plugin-starter.sanity.build

perf-studio – ./

🔍 Inspect: https://vercel.com/sanity-io/perf-studio/GYGr236sFkmX3QHmRkNjXR8Qx4Y1
✅ Preview: https://perf-studio-git-plain-custom-input-plugin-starter.sanity.build

[rexxars]

Closing this as we're getting closer to releasing v3 with a new plugin API.