class RectModification: Modification
placeable: {entity: Entity, factory: fun(): Entity}
topLeft: Vector2
bottomRight: Vector2
__new(
    self: RectModification,
    placeable: {entity: Entity, factory: fun(): Entity},
    topLeft: Vector2,
    bottomRight: Vector2,
    fillMode: any
)
fillMode: any
placeBoundaryCell(
    self: RectModification,
    attachable: SpectrumAttachable,
    x: number,
    y: number
)

Helper function to place a cell on the boundary