Restore skip-stack-frames-pattern setting with empty default

[huntie]

Summary

Fixes an unintended bug after applying #135, where removing the registration of the skip-stack-frames-pattern setting would cause DevTools to crash on start when Local Storage is cleared.

• Restores setting, instead giving an empty string default value. (This is the best equivalent we can give to mean null/undefined, and is evaluated here as an empty array when processed.)
• Adds Local Storage backwards compatibility checkbox to PR template.

Test plan

• Between builds, clear client-side storage via DevTools > Application > Clear site data.

Before	After
❌ DevTools fails to load (blank screen) with "No setting registered" error	✅ DevTools loads, no missing setting error
	✅ Dedicated settings panel renders and new patterns can be added

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.