status_bar: Add encoding indicator

[tomopumipumi]

Context / Related PRs This PR is the third part of the encoding support improvements, following:

• Support opening and saving files with legacy encodings #44819: Introduced initial legacy encoding support (Shift-JIS, etc.).
• Correct UTF-16 saving and add heuristic encoding detection #45243: Fixed UTF-16 saving behavior and improved binary detection.

Summary

This PR implements a status bar item that displays the character encoding of the active buffer (e.g., UTF-8, Shift_JIS). It provides visibility into the file's encoding and indicates the presence of a Byte Order Mark (BOM).

Features

• Encoding Indicator: Displays the encoding name in the status bar.
• BOM Support: Appends (BOM) to the encoding name if a BOM is detected (e.g., UTF-8 (BOM)).
• Configuration: Added active_encoding_button to status_bar settings to toggle visibility (default: true).
• Settings UI: Added the toggle switch to the Settings UI.
• Documentation: Updated configuring-zed.md and visual-customization.md.

Implementation Details

• Created ActiveBufferEncoding component in crates/encoding_selector.
• The click handler for the button is currently a no-op. Implementing the functionality to reopen files with a specific encoding has potential implications for real-time collaboration (e.g., syncing buffer interpretation across peers). Therefore, this PR focuses strictly on the visualization and configuration aspects to keep the scope simple and focused.
• Updated schema and default settings to include active_encoding_button.

Screenshots

Configuration

To hide the button, add the following to settings.json:

"status_bar": {
  "active_encoding_button": false
}

Heuristic Limitations:

The underlying detection logic (implemented in #44819 and #45243) prioritizes UTF-8 opening performance and does not guarantee perfect detection for all encodings. We consider this margin of error acceptable, similar to the behavior seen in VS Code. A future "Reopen with Encoding" feature would serve as the primary fallback for any misdetections.

Release Notes:

• Added a status bar item to display the active file's character encoding (e.g. UTF-8) and BOM status.