Added support for holidays in Nigeria

src/Countries/Nigeria.php

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+<?php
+
+namespace Spatie\Holidays\Countries;
+
+use Carbon\CarbonImmutable;
+
+class Nigeria extends Country
+{
+    public function countryCode(): string
+    {
+        return 'ng';
+    }
+
+    /** @return array<string, CarbonImmutable> */
+    protected function allHolidays(int $year): array
+    {
+        return array_merge([
+            'New Year\'s Day' => "01-01",
+            'Worker\'s Day' => "05-01",
+            'Children\'s Day' => "05-27",
+            'Democracy Day' => "06-12", 
+            'Independence Day' => "10-01",
+            'Christmas Day' => "12-25",
+            'Boxing Day' => "12-26",
+        ], $this->variableHolidays($year));
+    }
+
+    /** @return array<string, CarbonImmutable> */
+    protected function variableHolidays(int $year): array
+    {
+        $easter = CarbonImmutable::createFromTimestamp(easter_date($year))
+            ->setTimezone('Africa/Lagos');
+
+        return [
+            'Good Friday' => $easter->subDay(2),
+            'Easter Monday' => $easter->addDay(1),
+        ];
+    }
+}

tests/.pest/snapshots/Countries/NigeriaTest/it_can_calculate_nigerian_holidays.snap

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+[
+    {
+        "name": "New Year's Day",
+        "date": "2024-01-01"
+    },
+    {
+        "name": "Good Friday",
+        "date": "2024-03-29"
+    },
+    {
+        "name": "Easter Monday",
+        "date": "2024-04-01"
+    },
+    {
+        "name": "Worker's Day",
+        "date": "2024-05-01"
+    },
+    {
+        "name": "Children's Day",
+        "date": "2024-05-27"
+    },
+    {
+        "name": "Democracy Day",
+        "date": "2024-06-12"
+    },
+    {
+        "name": "Independence Day",
+        "date": "2024-10-01"
+    },
+    {
+        "name": "Christmas Day",
+        "date": "2024-12-25"
+    },
+    {
+        "name": "Boxing Day",
+        "date": "2024-12-26"
+    }
+]

tests/Countries/NigeriaTest.php

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+<?php
+
+namespace Spatie\Holidays\Tests\Countries;
+
+use Carbon\CarbonImmutable;
+use Spatie\Holidays\Holidays;
+
+it('can calculate nigerian holidays', function () {
+    CarbonImmutable::setTestNowAndTimezone('2024-01-01');
+
+    $holidays = Holidays::for(country: 'ng')->get();
+
+    expect($holidays)
+        ->toBeArray()
+        ->not()->toBeEmpty();
+
+    expect(formatDates($holidays))->toMatchSnapshot();
+});