Add ability to search local items saved in Playlist

[kylehickinson]

Some user feedback has come back asking to search videos they've saved in Playlist, we have some ongoing design for this in Figma: https://www.figma.com/design/gafcumKE9O6T4A844I8SvL/iOS-Playlist?node-id=5782-136088

This would be part 1 which adds just the ability to search for videos you have saved already

[Uni-verse]

Verified on iPhone 12 running iOS 18.2 using version 1.75 (102)

Verified the following acceptance criteria using test plan in brave/brave-core#26933 (comment)

• Verified tapping on the spyglass will open search field
• Verified you can search for items in multiple folders by name
• Verified items shown show the correct download states
• Verified long pressing on items shows the same context menu and that those buttons work correctly
• Verified that tapping on an item starts playing that item the same way as if tapped on the main sidebar/drawer list

example	example	example	example