Auto generate dark mode previews and screenshots (#776)

With this change, composable previews and screenshots should be created with just:
```
@ElementPreviews
@Composable
fun MyViewPreview() {
    ElementPreview { 
        MyView()
    }
}
```

- Adds `@ElementPreviews` which is a shorthand for:
```
@Preview(name = "D")
@Preview(name = "N", uiMode = Configuration.UI_MODE_NIGHT_YES)
```
Should be used in connection with the now public `fun ElementPreview()` composable.

- Adds ElementPreviews to previewAnnotations in dangerfile
- Screenshots of night mode previews are now rendered with night mode
- Replaces `ElementPreviewLight` and `ElementThemedPreview` with `ElementPreview`
- Deprecates `ElementPreviewDark` which should be removed.
- Remaining usages of `ElementPreviewDark` are now ignored during screenshot tests

libraries/featureflag/ui/src/main/kotlin/io/element/android/libraries/featureflag/ui/FeatureListView.kt

@@ -19,10 +19,10 @@ package io.element.android.libraries.featureflag.ui
 import androidx.compose.foundation.layout.Column
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
-import androidx.compose.ui.tooling.preview.Preview
+import io.element.android.libraries.designsystem.preview.ElementPreviews
 import io.element.android.libraries.designsystem.components.preferences.PreferenceCheckbox
+import io.element.android.libraries.designsystem.preview.ElementPreview
 import io.element.android.libraries.designsystem.preview.ElementPreviewDark
-import io.element.android.libraries.designsystem.preview.ElementPreviewLight
 import io.element.android.libraries.featureflag.ui.model.FeatureUiModel
 import io.element.android.libraries.featureflag.ui.model.aFeatureUiModelList
 import kotlinx.collections.immutable.ImmutableList
@@ -60,12 +60,13 @@ fun FeaturePreferenceView(
     )
 }
 
-@Preview
+@ElementPreviews
 @Composable
-internal fun FeatureListViewLightPreview() =
-    ElementPreviewLight { ContentToPreview() }
+internal fun FeatureListViewLightPreview() {
+    ElementPreview { ContentToPreview() }
+}
 
-@Preview
+@ElementPreviews
 @Composable
 internal fun FeatureListViewDarkPreview() =
     ElementPreviewDark { ContentToPreview() }