Extend copy mode keys with WORD movements and other bindings

[MartyMcFlyInTheSky]

I like copy mode and I usually use vim bindings. What I'm missing in wezterm's copy mode is for the WORD (big word) movements, e.g. W, B, E commands to work. Also I'm missing gm which moves the cursor to the center of the line.

[bew]

There is #5997 for WORD movements 🚀 Feel free to test it and report how it works for you!

For gm, nice that's one I didn't know! As for impl, this isn't a trivial addition since we don't easily have support for chained keys like this (we have key tables to kinda emulate this, but it's not easy..)