feat: add First and FirstN functions (#44)

- Add First: returns first element or error if empty
- Add FirstN: returns first n elements safely
- ErrEmptySlice error for consistent error handling
- Comprehensive tests including edge cases
- Benchmarks included

Resolves Issue 16

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

first.go

@@ -0,0 +1,33 @@
+package underscore
+
+import "errors"
+
+// ErrEmptySlice is returned when trying to get the first element of an empty slice
+var ErrEmptySlice = errors.New("underscore: empty slice")
+
+// First returns the first element of the slice.
+// Returns an error if the slice is empty.
+func First[T any](values []T) (T, error) {
+	var zero T
+	if len(values) == 0 {
+		return zero, ErrEmptySlice
+	}
+	return values[0], nil
+}
+
+// FirstN returns the first n elements of the slice.
+// If n is greater than the slice length, returns the entire slice.
+// If n is less than or equal to 0, returns an empty slice.
+func FirstN[T any](values []T, n int) []T {
+	if n <= 0 {
+		return []T{}
+	}
+	if n >= len(values) {
+		res := make([]T, len(values))
+		copy(res, values)
+		return res
+	}
+	res := make([]T, n)
+	copy(res, values[:n])
+	return res
+}

first_test.go

@@ -0,0 +1,97 @@
+package underscore_test
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	u "github.com/rjNemo/underscore"
+)
+
+func TestFirst(t *testing.T) {
+	nums := []int{1, 2, 3, 4, 5}
+	result, err := u.First(nums)
+	assert.NoError(t, err)
+	assert.Equal(t, 1, result)
+}
+
+func TestFirstEmpty(t *testing.T) {
+	_, err := u.First([]int{})
+	assert.Error(t, err)
+	assert.True(t, errors.Is(err, u.ErrEmptySlice))
+}
+
+func TestFirstSingleElement(t *testing.T) {
+	result, err := u.First([]int{42})
+	assert.NoError(t, err)
+	assert.Equal(t, 42, result)
+}
+
+func TestFirstStrings(t *testing.T) {
+	words := []string{"hello", "world"}
+	result, err := u.First(words)
+	assert.NoError(t, err)
+	assert.Equal(t, "hello", result)
+}
+
+func TestFirstN(t *testing.T) {
+	nums := []int{1, 2, 3, 4, 5}
+	result := u.FirstN(nums, 3)
+	assert.Equal(t, []int{1, 2, 3}, result)
+}
+
+func TestFirstNEmpty(t *testing.T) {
+	result := u.FirstN([]int{}, 3)
+	assert.Equal(t, []int{}, result)
+}
+
+func TestFirstNZero(t *testing.T) {
+	nums := []int{1, 2, 3}
+	result := u.FirstN(nums, 0)
+	assert.Equal(t, []int{}, result)
+}
+
+func TestFirstNNegative(t *testing.T) {
+	nums := []int{1, 2, 3}
+	result := u.FirstN(nums, -5)
+	assert.Equal(t, []int{}, result)
+}
+
+func TestFirstNGreaterThanLength(t *testing.T) {
+	nums := []int{1, 2, 3}
+	result := u.FirstN(nums, 10)
+	assert.Equal(t, []int{1, 2, 3}, result)
+}
+
+func TestFirstNSingleElement(t *testing.T) {
+	result := u.FirstN([]int{42}, 1)
+	assert.Equal(t, []int{42}, result)
+}
+
+func TestFirstNAll(t *testing.T) {
+	nums := []int{1, 2, 3}
+	result := u.FirstN(nums, 3)
+	assert.Equal(t, []int{1, 2, 3}, result)
+}
+
+func BenchmarkFirst(b *testing.B) {
+	nums := []int{1, 2, 3, 4, 5}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		u.First(nums)
+	}
+}
+
+func BenchmarkFirstN(b *testing.B) {
+	nums := make([]int, 1000)
+	for i := range nums {
+		nums[i] = i
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		u.FirstN(nums, 100)
+	}
+}