Add .metro-health-check to .gitignore

Summary:
Changelog:
[General][Changed] - Add Metro health check files to the template's `.gitignore`

We're adding an opt-in watcher health check to Metro, which can occasionally leave files named `.metro-health-check*` in the project. To ensure this doesn't clutter people's repos accidentally if enabled, we preemptively add this as a pattern to `.gitignore` in the new project template. We also add it to the React Native repo's `.gitignore` file just in case.

Reviewed By: arushikesarwani94

Differential Revision: D40352040

fbshipit-source-id: 261803b684d79292c014205084e61d5e4f7aeb3d

.gitignore

@@ -129,3 +129,6 @@ package-lock.json
 
 # Android memory profiler files
 *.hprof
+
+# Temporary files created by Metro to check the health of the file watcher
+.metro-health-check*

template/_gitignore

@@ -58,3 +58,6 @@ yarn-error.log
 # Ruby / CocoaPods
 /ios/Pods/
 /vendor/bundle/
+
+# Temporary files created by Metro to check the health of the file watcher
+.metro-health-check*