Currently, when SortMapKeys is set to true in Config, map keys are sorted using a standard alphanumeric sort. It should be possible to specify a custom map key sorting function to use, in order to prioritize certain map keys over others.

Example: Standard SortMapKeys behavior:

cfg := jsoniter.Config{
   SortMapKeys: true,
}.Froze()

m := map[string]int{
   "beta": 2,
   "alpha": 1,
}
output, _ := cfg.MarshalToString(m)

==> {"alpha":1,"beta":2}

Example: custom SortMapKeys behavior:

cfg := jsoniter.Config{
   SortMapKeys: true,
}.Froze()
// Desired --- register an extension that defines a custom map key sorting function
cfg.RegisterExtension(...some extension...)

m := map[string]int{
   "trump": "should be first",
   "beta": 2,
   "alpha": 1,
}
output, _ := cfg.MarshalToString(m)

// DESIRED OUTPUT
==> {"trump":"should be first","alpha":1,"beta":2}