Extract Flow types and constants (#49614)

Summary:
This change refactors the script to prebuild ios dependencies by:
- factoring out the constants
- factoring out the flow type definitions
- factoring out the .gitignore

bypass-github-export-check

## Changelog:

[INTERNAL] - Factor out flow types, constants and gitignore

Pull Request resolved: https://github.com/facebook/react-native/pull/49614

Test Plan:
Tested in this `AppDelegate.mm`:

Imports:

```obj-c
#import "AppDelegate.h"
#import <glog/logging.h>
#import <double-conversion.h>
#include <fmt/core.h>
#include <boost/assert.hpp>
#include <fast_float/fast_float.h>
#include <string>
#import <SocketRocket/SRWebSocket.h>
```

Code:
```obj-c
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  std::string input = "3.1416 xyz ";
  double_conversion::DoubleToStringConverter::EcmaScriptConverter();
  LOG(INFO) << "Hello from GLOG";
  fmt::print("Hello, world from FMT!\n");
  BOOST_ASSERT(100 == 100);
  double result;
  fast_float::from_chars(input.data(), input.data() + input.size(), result);
  LOG(INFO) << "Answer :" << result;

  NSArray *frameworks = [NSBundle allFrameworks];

  for (NSBundle *framework in frameworks) {
    NSString *frameworkName = framework.bundleURL.lastPathComponent;
    if ([frameworkName isEqualToString: @"ReactNativeDependencies.framework"]) {
      NSBundle *bundle = [NSBundle bundleWithURL:[framework bundleURL]];
      NSURL *bundleURL = [bundle URLForResource:@"ReactNativeDependencies_glog" withExtension:@"bundle"];
      NSBundle *resourceBundle = [NSBundle bundleWithURL:bundleURL];
      NSURL* url = [resourceBundle URLForResource:@"PrivacyInfo" withExtension:@"xcprivacy"];
      if (url == nil) {
        throw [NSException exceptionWithName:@"ResourceNotFoundException"
                                       reason:@"Could not find PrivacyInfo.xcprivacy in ReactNativeDependencies_glog bundle"
                                     userInfo:nil];
      }
      break;
    }
  }
  return YES;
}
```

Reviewed By: cortinico

Differential Revision: D70172536

Pulled By: cipolleschi

fbshipit-source-id: 91589693fd24e2b0d6d67759fb9fbc86841f4d13