`set-repl-handler!` helps with opening a repl when a particular mode is
active in a buffer. We want to be able to open a `sly-mrepl` whenever we
are in a lisp buffer, so we should have the repl handler look for
`'lisp-mode` to define opening a sly repl.
I assume the lookup handlers should be making sure we are in a lisp
buffer (similar reasoning to the repl-handler).
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