fix: Do not omit alt on getImgProps return type, ImgProps (#70608)

### What?

Small type tweak, asked for in
#70443

### Why?

The `alt` prop is required on the call to `getImgProps`, and the
implementation, returns it back to the caller, however, the type
signature of the return type for `getImgProps`, omits the `alt` prop,
which means that users have to manually apply the `alt` prop.

That is to say, that in this example, tweaked from the docs:

```jsx
import { getImageProps } from 'next/image'
 
export default function Page() {
  const common = { alt: 'Theme Example', width: 800, height: 400 }
  const {
    props: { srcSet: dark },
  } = getImageProps({ ...common, src: '/dark.png' })
  const {
    props: { srcSet: light, ...rest },
  } = getImageProps({ ...common, src: '/light.png' })
 
  return (
    <picture>
      <source media="(prefers-color-scheme: dark)" srcSet={dark} />
      <source media="(prefers-color-scheme: light)" srcSet={light} />
      {/* the alt prop is here, but as far as TypeScript is concerned, it is not */}
      <SomeCustomImageComponent {...rest} /> 
    </picture>
  )
}
``` 

So users have to do:

```jsx
      <SomeCustomImageComponent {...rest} alt={common.alt} /> 
```

See more at, #70443

### How?

Remove `alt` from the Omit union.

```tsx
export type ImgProps = Omit<ImageProps, 'src'  | 'alt' | 'loader'> & { /* other stuff */ }
```

Fixes #70443

packages/next/src/shared/lib/get-img-props.ts

@@ -69,7 +69,7 @@ export type ImageProps = Omit<
   lazyRoot?: string
 }
 
-export type ImgProps = Omit<ImageProps, 'src' | 'alt' | 'loader'> & {
+export type ImgProps = Omit<ImageProps, 'src' | 'loader'> & {
   loading: LoadingValue
   width: number | undefined
   height: number | undefined

test/unit/next-image-get-img-props.test.ts

@@ -37,6 +37,30 @@ describe('getImageProps()', () => {
       ['src', '/_next/image?url=%2Ftest.png&w=256&q=75'],
     ])
   })
+
+  it('should have correct type for props', async () => {
+    const { props } = getImageProps({
+      alt: 'a nice desc',
+      id: 'my-image',
+      src: '/test.png',
+      width: 100,
+      height: 200,
+    })
+
+    expect(props.alt).toBeString()
+    expect(props.id).toBeString()
+    expect(props.loading).toBeString()
+
+    expect(props.width).toBeNumber()
+    expect(props.height).toBeNumber()
+
+    expect(props.decoding).toBeString()
+    expect(props.style).toBeObject()
+    expect(props.style.color).toBeString()
+    expect(props.src).toBeString()
+    expect(props.srcSet).toBeString()
+  })
+
   it('should handle priority', async () => {
     const { props } = getImageProps({
       alt: 'a nice desc',