Better UX: Add a 'new file/folder' button on folder hover in file explorer

https://github.com/user-attachments/assets/e818b6a9-7237-4452-bd6e-d611edd7cbc6

## Problem
Currently, adding a file/folder requires right-click → New File, which is 3 clicks.
Or if you had to use the current 'New File...' button, u have to select your desired folder,
which would take at most two clicks, and this causes annoying mouse traveling each time.

## Solution
Show a '+' button when hovering a folder in the Explorer view.
Clicking it opens an inline input to name the new file/folder.

## Benefit
- Reduces clicks from 3 or 2 to 1
- The old button is on top of screen, while the desired folder could be on bottom, that slows down devs
- Improves discoverability
- Consistent with existing hover actions in VS Code