gdb/python: introduce gdb.warning() function

gdb/python: introduce gdb.warning() function

This commit adds a new gdb.warning() function.  This function takes a
string and then calls GDB's internal warning() function.  This will
display the string as a warning.

Using gdb.warning() means that the message will get the new emoji
prefix if the user has that feature turned on.  Also, the message will
be sent to gdb.STDERR without the user having to remember to print to
the correct stream.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>