Fabric: Add Unicode prefix to AttachmentCharacter (#28617)

Summary:
This pull request adds a Unicode `u8` prefix to the string literal returned in `AttributedString.cpp`'s `Fragment::AttachmentCharacter()`.

This fixes the following error when building on MSVC:

```
react\attributedstring\AttributedString.cpp(21): error C4566: character represented by universal-character-name '\uFFFC' cannot be represented in the current code page (1252)
```

## Changelog

[Internal] [Fixed] - Fabric: Add Unicode prefix to AttachmentCharacter
Pull Request resolved: https://github.com/facebook/react-native/pull/28617

Test Plan: The Fabric test suite has been ran on a Clang-based build of Fabric on macOS, and no regressions in it have been noted.

Differential Revision: D21118078

Pulled By: shergin

fbshipit-source-id: c105de5e4edb67fed97ce44153a75d9d380bf588

ReactCommon/fabric/attributedstring/AttributedString.cpp

@@ -18,7 +18,7 @@ using Fragments = AttributedString::Fragments;
 #pragma mark - Fragment
 
 std::string Fragment::AttachmentCharacter() {
-  return "\uFFFC"; // Unicode `OBJECT REPLACEMENT CHARACTER`
+  return u8"\uFFFC"; // Unicode `OBJECT REPLACEMENT CHARACTER`
 }
 
 bool Fragment::isAttachment() const {