Convert a Japanese era date (wareki) to a Gregorian date / 和暦から西暦

Convert a Japanese era date to YYYY-MM-DD. Accepts 令和8年10月7日, 令和元年5月1日, R8.10.7, H31/4/30, Reiwa 8.10.7, kanji numerals (昭和六十四年一月七日), a trailing weekday such as （水） or 水曜日, and full-width characters. input_era/input_era_year echo the input; actual is the correct era for that date. If the date is outside the input era (e.g. 平成31年5月1日, written after the era changed), it is still converted by counting years, within_era is false and flags has outside_era. If a weekday was given and does not match, weekday_matches is false and flags has weekday_mismatch. / 和暦の日付を西暦にする。元号の範囲の外（平成31年5月1日など）も年を数えて変換し、within_era が false・flags に outside_era。曜日が合わなければ weekday_mismatch。