NormalizerNFKC: add unify_katakana_du_small_sounds option

[HashidaTKS]

When unify_katakana_du_small_sounds is specified, NormalizerNFKC* normalize characters as below.

ヅァ -> ザ
ヅィ -> ジ
ヅェ -> ゼ
ヅォ -> ゾ

Usage:

normalize \
  'NormalizerNFKC130("unify_katakana_du_small_sounds", true, \
                     "report_source_offset", true)' \
  "ヅァヅィヅェヅォ" \
  WITH_CHECKS|WITH_TYPES

[kou]

Ah, we should have used du_sounds, gu_sounds and zu_sounds for option name...

[HashidaTKS]

Ah, we should have used du_sounds, gu_sounds and zu_sounds for option name...

Hmm, if d_sounds is renamed to du_sounds and z_sounds to zu_sounds, what should be named a normalizer for ヅ to ズ?
Should du_sounds also normalize ヅ to ズ?

[kou]

what should be named a normalizer for ヅ to ズ?

du_sound?

Should du_sounds also normalize ヅ to ズ?

Hmm. du_small_sounds?

[HashidaTKS]

what should be named a normalizer for ヅ to ズ?

du_sound?

Should du_sounds also normalize ヅ to ズ?

Hmm. du_small_sounds?

Thanks!
By apply your comments, I would like to name them as below.

• This pull request: du_small_sounds
• The normalizer for ヅ to ズ: du_sound

[HashidaTKS]

Renamed.
I will rename g_sounds to gu_sounds and z_sounds to zu_sounds as following task (in other pull requests).