fix(assets): fix getImage options type

[norskeld]

Changes

This PR fixes #12319

As I mentioned in the issue, there are two ways of fixing the problem:

1. Use a type helper to properly handle Omitting from types containing index signatures.
2. Remove that index signature from the ImageTransform.

I went with the first one since I guess that index signature was put there for a reason. :)

I also applied Simplify. This is to force TypeScript better resolve properties. Not doing that results in the following error when an empty options objects is passed to getImage:

Type '{}' is not assignable to type 'UnresolvedImageTransform'.
  Property 'src' is missing in type '{}' but required in type '{ src: string | ImageMetadata | Promise<{ default: ImageMetadata; }>; inferSize?: boolean | undefined; }'.ts(2322)

With Simplify the error is:

Type '{}' is not assignable to type 'UnresolvedImageTransform'.
  Property 'src' is missing in type '{}' but required in type '{ [x: string]: any; width?: number | undefined; widths?: number[] | undefined; densities?: (number | `${number}x`)[] | undefined; height?: number | undefined; quality?: ImageQuality | undefined; format?: ImageOutputFormat | undefined; src: string | ... 1 more ... | Promise<...>; inferSize?: boolean | undefined; }'.

The latter is rather verbose, but I'd prefer this over the former's actually misleading message.

Testing

Not sure how to add non-fragile or hardcoded tests here.

Docs

This is a fix that doesn't require any changes in docs.

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Latest commit: 84426ca

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[Princesseuh]

Looks great to me! Thank you for contributing!