Fix get_highlight_source line splitting

TL;DR: With .splitlines() we splitted over universal splitlines which did not correspond to GDB's target code line splitting...

As a result we got `context code` to produce bogus out of sync lines that didn't correspond to GDB's `line` command.

See also https://docs.python.org/3/library/stdtypes.html#str.splitlines

pwndbg/commands/context.py

@@ -457,7 +457,7 @@ def get_highlight_source(filename):
     if pwndbg.config.syntax_highlight:
         source = H.syntax_highlight(source, filename)
 
-    source_lines = source.splitlines()
+    source_lines = source.split('\n')
     source_lines = tuple(line.rstrip() for line in source_lines)
     return source_lines