refactor: split copier.go into smaller files

Split monolithic copier.go (1077 lines) into focused modules:
- copier.go: Core framework (95 lines)
- copy_value.go: Copy functions and type handling (858 lines)
- reflect.go: Reflection utilities (100 lines)
- converter.go: Type converter (38 lines)
- options.go: Configuration (renamed from optiongs.go)

Bug fixes:
- Fixed string to *string copying (empty strings now work)
- Fixed *time.Time to *time.Time copying
- Fixed nil slice handling
- Fixed copyStructToSlice pointer element handling

Tests: Fixed critical data copy issues. Remaining method-mapping
features require additional implementation.

converter.go

@@ -0,0 +1,38 @@
+package copier
+
+import "reflect"
+
+func (c *copier) lookupAndCopyWithConverter(dst, src reflect.Value, fieldName string) (bool, error) {
+	if cnv, ok := c.opt.convertByName[fieldName]; ok {
+		return convert(dst, src, cnv)
+	}
+
+	if len(c.opt.converters) > 0 {
+		pair := converterPair{
+			SrcType: src.Type(),
+			DstType: dst.Type(),
+		}
+
+		if cnv, ok := c.opt.converters[pair]; ok {
+			return convert(dst, src, cnv)
+		}
+	}
+
+	return false, nil
+}
+
+func convert(dst, src reflect.Value, fn convertFunc) (bool, error) {
+	result, err := fn(src.Interface())
+	if err != nil {
+		return false, err
+	}
+
+	if result != nil {
+		dst.Set(reflect.ValueOf(result))
+	} else {
+		dst.Set(reflect.Zero(dst.Type()))
+	}
+
+	return true, nil
+
+}