Add script to generate native modules specs with react-native-codegen

Summary:
Adds a script that uses `react-native-codegen` to generate FBReactNativeSpec.
The generated output should not be considered ready for production use at this time.
The goal of adding this script at this time is to demonstrate the current status of native modules specs code generation in open source.

For example, the generated output may be used in RNTester, with some modifications due to some naming differences in react-native-codegen's output when compared to the FBReactNativeSpec files generated by the old codegen.

Usage:

```
./scripts/generate-native-modules-specs.sh ./codegen-out
```

Changelog: [Internal]

Reviewed By: TheSavior

Differential Revision: D21471004

fbshipit-source-id: 5ff3c57807d9ba2c91dc7fe32d227d218732b059

.gitignore

@@ -96,6 +96,7 @@ RNTester/Pods/*
 # react-native-codegen
 /ReactCommon/fabric/components/rncore/
 /schema-rncore.json
+/schema-native-modules.json
 
 # Visual studio
 .vscode

scripts/generate-native-modules-specs.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+# This script collects the JavaScript spec definitions for native
+# modules, then uses react-native-codegen to generate native code.
+# The script will copy the generated code to the final location by
+# default. Optionally, call the script with a path to the desired
+# output location.
+#
+# Usage:
+#   ./scripts/generate-native-modules-specs.sh [output-dir]
+#
+# Example:
+#   ./scripts/generate-native-modules-specs.sh ./codegen-out
+
+# shellcheck disable=SC2038
+
+set -e
+
+describe () {
+  printf "\\n\\n>>>>> %s\\n\\n\\n" "$1"
+}
+
+THIS_DIR=$(cd -P "$(dirname "$(readlink "${BASH_SOURCE[0]}" || echo "${BASH_SOURCE[0]}")")" && pwd)
+RN_DIR=$(cd "$THIS_DIR/.." && pwd)
+
+TEMP_DIR=$(mktemp -d /tmp/react-native-codegen-XXXXXXXX)
+
+SRCS_DIR=$(cd "$RN_DIR/Libraries" && pwd)
+LIBRARY_NAME="FBReactNativeSpec"
+MODULE_SPEC_NAME="FBReactNativeSpec"
+
+SCHEMA_FILE="$RN_DIR/schema-native-modules.json"
+OUTPUT_DIR="$SRCS_DIR/$LIBRARY_NAME/$MODULE_SPEC_NAME"
+
+describe "Generating schema from flow types"
+grep --exclude NativeUIManager.js --include=Native\*.js -rnwl "$SRCS_DIR" -e 'export interface Spec extends TurboModule' -e "export default \(TurboModuleRegistry.get(Enforcing)?<Spec>\('.*\): Spec\);/" \
+  | xargs yarn flow-node packages/react-native-codegen/src/cli/combine/combine-js-to-schema-cli.js "$SCHEMA_FILE"
+
+describe "Generating native code from schema"
+yarn flow-node packages/react-native-codegen/buck_tests/generate-tests.js "$SCHEMA_FILE" "$LIBRARY_NAME" "$TEMP_DIR" "$MODULE_SPEC_NAME"
+
+if [ -n "$1" ]; then
+  OUTPUT_DIR="$1"
+  mkdir -p "$OUTPUT_DIR"
+fi
+
+describe "Copying $MODULE_SPEC_NAME output  to $OUTPUT_DIR"
+cp "$TEMP_DIR"/$MODULE_SPEC_NAME* "$OUTPUT_DIR/."