Cannot use `@EmergeSnapshotConfig` annotation with custom multipreview annotations

[rbro112]

Due to the AnnotationTarget.FUNCTION target on EmergeSnapshotConfig, EmergeSnapshotConfig cannot be used with custom multipreview annotations.

Example of desired behavior:

@Preview(
    name = "1. Light Theme",
    ...
)
@Preview(
    name = "2. Dark Theme",
    ...
)
@EmergeSnapshotConfig(precision = 0.99f)
annotation class CustomPreview

@EmergeSnapshotConfig(ignore = true)
annotation class IgnoredEmptySnapshot

Both CustomPreview and IgnoredEmptySnapshot should apply the precision/ignore from EmergeSnapshotConfig respectively to any functions they're applied on.

We can implement this by updating the annotation target to be AnnotationTarget.ANNOTATION_CLASS. We'll need to add support to the Emerge backend to ensure the config is applied to all previews annotated with the multipreview annotation.