lunar: introduce lwtf and lcd functions

These functions have been incredibly helpful to me in the last years and I
think it's time to include them in the official lunar module.

Yes, there is `lvu what`, but `lwtf` is shorter and has more style :). Also,
it mimics the standard `wtf` tool.

Yes, there is `lvu cd` but that opens a non-interactive subshell for each use
of the function and also drops PS1 etc. Using a bash function for that enables
us to jump between many modules keeping the same shell environment.