Move ContainmentMode enum to public h3api.h.in

[ajfriend]

I think we need to move the ContainmentMode enum from polygon.h to the public API in h3api.h.in:

/**
 * Values representing polyfill containment modes, to be used in
 * the `flags` bit field.
 */
typedef enum {
    CONTAINMENT_CENTER = 0,       ///< Cell center is contained in the shape
    CONTAINMENT_FULL = 1,         ///< Cell is fully contained in the shape
    CONTAINMENT_OVERLAPPING = 2,  ///< Cell overlaps the shape at any point
    CONTAINMENT_OVERLAPPING_BBOX = 3,  ///< Cell bounding box overlaps shape
    CONTAINMENT_INVALID = 4  ///< This mode is invalid and should not be used
} ContainmentMode;

[ajfriend]

Should we also indicate that it is experimental in the naming?