Consider renaming NodeImageMode to ImageNodeMode

[mgi388]

• In 0.15, we now have ImageNode, and I just noticed it has a field image_mode of type NodeImageMode (notice the inconsistency):

pub struct ImageNode {
    /// Controls how the image is altered to fit within the layout and how 
    /// the layout algorithm determines the space to allocate for the image.
    pub image_mode: NodeImageMode,
}

• Looks like image_mode was added in Improved UiImage and Sprite scaling and slicing APIs #16088 and merged on 5 Nov.
• UiImage was renamed by cart in UiImage -> ImageNode, UiImageSize -> ImageNodeSize #16271 and merged on 8 Nov.
• It's possible the close timing here meant it was missed.
• I can't see NodeImageMode used by anything else that would suggest keeping this name was deliberate.