Fix possible truncated source while colorizing traceback in Python 3.12

In particular, a "\" at the end of a line (indicating line
continuation) would have been lost. This is because prior to Python
3.12, the tokenizer would emit "ERRORTOKEN" (as the continuation of the
line is not part of the traceback) but it would be colorized anyway.
With Python 3.12, a "TokenError" is raised instead, which cause the
token iteration to end prematurely. Because of a typo in the code, the
final part of the source (usually empty, except if it couldn't be
tokenized) wasn't added to the output string.

loguru/_better_exceptions.py

@@ -85,7 +85,7 @@ def highlight(self, source):
             _, end_column = end
 
             if start_row != row:
-                source = source[:column]
+                source = source[column:]
                 row, column = start_row, 0
 
             if type_ != tokenize.ENCODING: