[BUG] UnicodeDecodeError when using non-ASCII characters in files

When using tkreload with files containing non-ASCII characters (such as Unicode emojis or special characters), the library throws a UnicodeDecodeError because it uses the default 'charmap' codec (cp1252 on Windows) instead of UTF-8 for file operations.

**Error Message**: `UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 1221: character maps to `

This error occurs in the file handling within `tkreload.app_event_handler`.

## Steps to Reproduce
1. Install tkreload (`pip install tkreload`)
2. Create a Python file using CustomTkinter or Tkinter that contains Unicode characters (e.g., emojis)
3. Run the application with auto-reload enabled
4. The application will crash with a UnicodeDecodeError when tkreload tries to read the file

## Example Code That Triggers the Error
```python
import customtkinter as ctk

app = ctk.CTk()
# Using an emoji or special character in the UI
label = ctk.CTkLabel(app, text="☀️") # This will trigger the error
app.mainloop()
```

**Current Behavior**

- tkreload uses the default system encoding (cp1252 on Windows)
- Files containing non-ASCII characters cause the application to crash
- No built-in handling for UTF-8 encoded files

**Expected Behavior**

- tkreload should handle UTF-8 encoded files correctly
- Support for Unicode characters in source files
- Graceful handling of different file encodings

**Workaround**
Currently, this can be worked around by monkey-patching the file opening operation in tkreload:
```python
import tkreload.app_event_handler

original_open = tkreload.app_event_handler.open

def utf8_open(file_path, mode='r', *args, **kwargs):
return original_open(file_path, mode, encoding='utf-8', *args, **kwargs)

tkreload.app_event_handler.open = utf8_open
```

**Proposed Fix**
The library should be updated to:
- Use UTF-8 as the default encoding for file operations
- Add an optional configuration parameter for specifying the desired encoding
- Implement proper error handling for encoding issues

**Environment**

- Python version: 3.11
- tkreload version: 1.0.4
- OS: Windows
- Related packages: CustomTkinter 5.2.2

**Additional Notes**
This issue particularly affects applications using Unicode characters, emojis, or international text. The current workaround requires additional setup code that should ideally be handled by the library itself.