Fix for failure in bracketed paste in gpt modules

Moderate usage of gpt #38 and sgpt #37 have revealed a pasting bug for long prompts. This does not occur for ipython, so this is an interaction between the REPL implementations of at least gpt and sgpt and vim terminal. For a long prompt, the first roughly 30 lines are pasted correctly, but then lines after that are fed into the interpreter one at a time. My suspicion is that this is associated with the buffering of [vim](https://github.com/vim/vim/blob/947f752a47b107529b5d591e4e24b51237c20497/src/channel.c#L4238) and the REPL terminal implementations. One solution, proposed here, to use additional start and end bracketing text, forcing the REPL to enter multiline mode, which extends (if necessary) the bracketed paste.