ot::Mac::ChannelMask::ToString() const method

Converts the channel mask into a human-readable string. Examples of possible output: - empty mask -> "{ }" - all channels -> "{ 11-26 }" - single channel -> "{ 20 }" - multiple ranges -> "{ 11, 14-17, 20-22, 24, 25 }" - no range -> "{ 14, 21, 26 }"