fix UnicodeEncodeError

[3mora2]

error ->

Traceback (most recent call last):
  File "E:\Projects\Python\QssStylesheetEditor\src\ui\mainwin.py", line 540, in save
    self.saveAs()
  File "E:\Projects\Python\QssStylesheetEditor\src\ui\mainwin.py", line 553, in saveAs
    self.editor.save(self.file.encode("utf-8"))
  File "E:\Projects\Python\QssStylesheetEditor\venvtest\Lib\site-packages\CodeEditor\editor.py", line 366, in save
    outfile.write(self.text())
  File "C:\Users\ammar\AppData\Local\Programs\Python\Python311\Lib\encodings\cp1252.py", line 19, in encode
    return codecs.charmap_encode(input,self.errors,encoding_table)[0]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeEncodeError: 'charmap' codec can't encode characters in position 513-517: character maps to <undefined>

please, update lib CodeEditor in editor.py -> Editor class -> save method:

    def save(self, filename):
        """Save the editor contents to the given filename."""
        with open(filename, 'w', newline='', encoding="utf-8") as outfile:
            # 不指定newline，则换行符为各系统默认的换行符（\n, \r, or \r\n, ）
            # newline=''表示不转换
            outfile.write(self.text())
            self.setModified(False)