add ignore patterns setting for `import/extensions`

When using modern front-end bundlers like [vite](https://vitejs.dev/), it is becoming more common to enable "virtual" files that may not actually exist in the file system.

For example, [this plugin](https://github.com/antfu/unplugin-icons) provides access to many open source icon libraries by importing files with a syntax like
```
import Arrow from '~icons/octicon/arrow-up-24';
```

I can fix the `import/no-unresolved` rule by creating an ignore pattern, however I don't see how I can fix this `import/extensions` rule because the settings only allow making changes for a specific extension, and these files are not found. Nor would I even want to know or care about what the extension is, I just want to say that those imports shouldn't require an extension.

Probably the best solution would be to provide the same kind of "ignore" setting that `import/no-unresolved` has available?

Start by reading the import/extensions rule and its settings alongside the import/no-unresolved rule's ignore setting. The work is done when virtual imports such as ~icons/octicon/arrow-up-24 can be exempted from extension requirements without specifying an extension.

Written by the indexing model from the issue text.