feat: add Init function (all but last) (#45)

- Add Init: returns all elements except last, and the last element
- Useful for destructuring lists from the right
- Comprehensive tests including edge cases
- Benchmark included

Example: Init([1,2,3,4,5]) → ([1,2,3,4], 5)

Resolves Issue 17

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

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

init.go

@@ -0,0 +1,18 @@
+package underscore
+
+// Init returns all elements except the last one, and the last element separately.
+// Returns an empty slice and zero value if the input slice is empty.
+// Also known as "uncons from the right" or "snoc" inverse.
+func Init[T any](values []T) ([]T, T) {
+	var last T
+	if len(values) == 0 {
+		return []T{}, last
+	}
+	if len(values) == 1 {
+		return []T{}, values[0]
+	}
+
+	res := make([]T, len(values)-1)
+	copy(res, values[:len(values)-1])
+	return res, values[len(values)-1]
+}

init_test.go

@@ -0,0 +1,65 @@
+package underscore_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	u "github.com/rjNemo/underscore"
+)
+
+func TestInit(t *testing.T) {
+	nums := []int{1, 2, 3, 4, 5}
+	init, last := u.Init(nums)
+	assert.Equal(t, []int{1, 2, 3, 4}, init)
+	assert.Equal(t, 5, last)
+}
+
+func TestInitEmpty(t *testing.T) {
+	init, last := u.Init([]int{})
+	assert.Equal(t, []int{}, init)
+	assert.Equal(t, 0, last)
+}
+
+func TestInitSingleElement(t *testing.T) {
+	init, last := u.Init([]int{42})
+	assert.Equal(t, []int{}, init)
+	assert.Equal(t, 42, last)
+}
+
+func TestInitTwoElements(t *testing.T) {
+	init, last := u.Init([]int{1, 2})
+	assert.Equal(t, []int{1}, init)
+	assert.Equal(t, 2, last)
+}
+
+func TestInitStrings(t *testing.T) {
+	words := []string{"hello", "world", "!"}
+	init, last := u.Init(words)
+	assert.Equal(t, []string{"hello", "world"}, init)
+	assert.Equal(t, "!", last)
+}
+
+func TestInitDoesNotMutate(t *testing.T) {
+	original := []int{1, 2, 3, 4, 5}
+	init, last := u.Init(original)
+
+	// Modify returned slice
+	init[0] = 999
+
+	// Original should be unchanged
+	assert.Equal(t, 1, original[0])
+	assert.Equal(t, 5, last)
+}
+
+func BenchmarkInit(b *testing.B) {
+	nums := make([]int, 1000)
+	for i := range nums {
+		nums[i] = i
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		u.Init(nums)
+	}
+}