Add country Japan (JP)

The following locale needs to be verified/completed:
```
class ja_JP(Locale):
"""
01-01: [NF] 元日
02-11: [NF] 建国記念の日
04-29: [NF] 昭和の日
05-03: [NF] 憲法記念日
05-04: [NF] みどりの日
05-05: [NF] こどもの日
11-03: [NF] 文化の日
11-23: [NF] 勤労感謝の日
12-23: [NF] 天皇誕生日
: [NV] 成人の日
: [NV] 春分の日
: [NV] 海の日
: [NV] 敬老の日
: [NV] 秋分の日
: [NV] 体育の日
"""

locale = "jp-JP"
```
Checklist:
* Are all holidays (i.e. days "which are defined by law on which business or work are suspended or reduced") listed? Can you provide the legal sources which define the holidays?
* Is the spelling correct? Can you maybe provide a translation?
* Are the fixed dates `-` correct?
* If a holiday has a variable date: Is the algorithm to calculate it correct/can you provide it if missing? If it depends on the easter date: Is it the western or orthodox easter date?
* If a holiday is proclaimed by the government: Can you provide the source?
* Are holidays moved to a different date, e.g. if they fall on a saturday/sunday/...?
* Are holidays observed on a different date (is there a substitute holiday), e.g. if they fall on a saturday/sunday/...?
* Are the flags correct? (`N` = national holiday, defined by law, `R` = holiday with religious background, `F` = holiday with fixed date, `V` = holiday with variable date)
* Have there been any holidays renamed/deleted/created since 2011? If yes, which ones and since when?

Start by reviewing the ja_JP(Locale) entry in the issue and verify each listed Japanese holiday against authoritative legal sources. Check names, translations, fixed and variable dates, substitute-day rules, flags, and changes since 2011. Done means the locale data is complete and every checklist question is supported by sources; no file or test is named in the issue.

Written by the indexing model from the issue text.