Add prefix filter for light/dark modes

[fedulvtubudul]

Our designer uses variable modes for light and dark modes. Nope, actually we have both variable modes AND color styles, and it seems to be irrelevant to this exact proposal.

figma-export sees these colours like this (breakpoint inside loadColorsFromSingleFile method):

(lldb) print colors.map({ $0.name }).sorted()
([String]) $R1 = 66 values {
  [0] = "dark-theme/accent-01"
  [1] = "dark-theme/accent-02"
  [2] = "dark-theme/accent-03"
...
  [33] = "light-theme/accent-01"
  [34] = "light-theme/accent-02"
  [35] = "light-theme/accent-03"
...

So I assume adding a new parameter so that we can filter resources not only by suffix, but by prefix as well.

[fedulvtubudul]

@subdan please check the PR #235 i've done regarding this request.