Support static next/image out of the box

[shuding]

This PR introduces a new option to the Nextra core unstable_staticImage: boolean, disabled by default, which transpiles the image node into the next/image component, only if the image url is a relative path (begins with ./ or ../).

For example:

![Logo](../../public/img.jpeg)

with Nextra, is equivalent to

import Image from 'next/image'
import img from '../../public/img.jpeg'

<Image src={img} alt="Logo" placeholder='blur'>

in MDX.

Notes

• It currently only supports "importable static images" (with a relative path) because you don't need to specify width & height, which is very convenient for writing Markdowns.
• It pre-configures options like layout and placeholder for you, since Nextra is opinionated and wants to support the most common cases with the best UX. To customize it, simply import next/image in MDX.
• Closes Improve img #104.

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