gb: add Coronation Day for King Charles III

Fixes #108

Author: rickar

v2/gb/gb_holidays.go

@@ -50,6 +50,17 @@ var (
 		EndYear:   2020,
 	}
 
+	// CoronationDay represents the Coroation Day for King Charles III on 8-May
+	CoronationDay = &cal.Holiday{
+		Name:      "Coronation of King Charles III",
+		Type:      cal.ObservanceBank,
+		Month:     time.May,
+		Day:       8,
+		Func:      cal.CalcDayOfMonth,
+		StartYear: 2023,
+		EndYear:   2023,
+	}
+
 	// SpringHoliday represents Spring Bank Holiday on the last Monday of May
 	SpringHoliday = &cal.Holiday{
 		Name:    "Spring Bank Holiday",
@@ -120,6 +131,7 @@ var (
 		EasterMonday,
 		EarlyMay,
 		VEDay,
+		CoronationDay,
 		SpringHoliday,
 		SpringHoliday2022,
 		PlatinumJubilee,

v2/gb/gb_holidays_test.go

@@ -65,6 +65,15 @@ func TestHolidays(t *testing.T) {
 		{VEDay, 2021, time.Time{}, time.Time{}},
 		{VEDay, 2022, time.Time{}, time.Time{}},
 
+		{CoronationDay, 2017, time.Time{}, time.Time{}},
+		{CoronationDay, 2018, time.Time{}, time.Time{}},
+		{CoronationDay, 2019, time.Time{}, time.Time{}},
+		{CoronationDay, 2020, time.Time{}, time.Time{}},
+		{CoronationDay, 2021, time.Time{}, time.Time{}},
+		{CoronationDay, 2022, time.Time{}, time.Time{}},
+		{CoronationDay, 2023, d(2023, 5, 8), d(2023, 5, 8)},
+		{CoronationDay, 2024, time.Time{}, time.Time{}},
+
 		{SpringHoliday, 2015, d(2015, 5, 25), d(2015, 5, 25)},
 		{SpringHoliday, 2016, d(2016, 5, 30), d(2016, 5, 30)},
 		{SpringHoliday, 2017, d(2017, 5, 29), d(2017, 5, 29)},