Add support for CUSTOM palette type for changeMode

In the UI, there is an extra option called CUSTOM. This lets me manually load an ACT or configure the palette and then run the change operation using that palette.
Currently, the UXP API has support for PREVIOUSPALETTE, but not CUSTOM. There are no functions that allow saving or loading an ACT file from UXP, nor is there support for modifying the palette prior to running a conversion using previous palette.
Being able to change the palette programmatically would allow for scripting large batch conversions. Sometimes, there can be a small set of palletes that must be used, or some subsets of exports must all use the same palette.
Adding support for either CUSTOM palette along with loading an ACT during use or allowing for the modification of the palette that would be used for the next PREVIOUSPALETTE operation would go a long way towards making mode conversion via script on par with manual operations for functionality supported.