Allow ReactCommon to be used in Swift libraries (DEFINES_MODULE)

Swift Pods require the use of [modular headers](https://blog.cocoapods.org/CocoaPods-1.5.0/) to be statically linked. To interop with Objective-C modules, you need to make the Objective-C module "define a Module", that is modular header export.

This is already the case for a few podspecs so they can be consumed in Swift libraries, but `ReactCommon` doesn't do this yet and therefore breaks in a few libraries of mine, for example see this issue: mrousavy/react-native-vision-camera#195.

If I were to include `ReactCommon` in my Swift library's podspec, the following error arises:

```
[!] The following Swift pods cannot yet be integrated as static libraries:

The Swift pod `VisionCamera` depends upon `RCT-Folly`, which does not define modules.
To opt into those targets generating module maps (which is necessary to import them from Swift
when building as static libraries), you may set `use_modular_headers!` globally in your Podfile, or
specify `:modular_headers => true` for particular dependencies.
```

So this PR fixes this issue by allowing Swift libraries to consume the `ReactCommon` podspec since it now exports modular headers.

ReactCommon/ReactCommon.podspec

@@ -34,8 +34,9 @@ Pod::Spec.new do |s|
  s.compiler_flags = folly_compiler_flags + ' ' + boost_compiler_flags
  s.pod_target_xcconfig = { "HEADER_SEARCH_PATHS" => "\"$(PODS_ROOT)/boost-for-react-native\" \"$(PODS_ROOT)/RCT-Folly\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/Headers/Private/React-Core\"",
  "USE_HEADERMAP" => "YES",
+ "DEFINES_MODULE" => "YES"
  "CLANG_CXX_LANGUAGE_STANDARD" => "c++14" }
-
+ 
  # TODO (T48588859): Restructure this target to align with dir structure: "react/nativemodule/..."
  # Note: Update this only when ready to minimize breaking changes.
  s.subspec "turbomodule" do |ss|