Add copy-mode-vi bindings for search keys

[liangkarl]

There is a use case that the user may jump in copy mode, which didn't have valid operation for this.

[ddzero2c]

Thanks for the contribution! Just to clarify — since tmux bind registers to the prefix table by default, prefix + s already works in copy mode without any extra configuration.

If you specifically need to trigger easymotion without the prefix key in copy mode, I'd suggest making it opt-in via a config option rather than changing the default, so existing users won't have their copy-mode-vi bindings overwritten unexpectedly. Could you share more about your use case?

[liangkarl]

Yes, using prefix + s is reasonable when the console isn't in any tmux mode. However, when tmux is already in copy mode, it would be a little weird to enforce using prefix + s. Just like the default keybinds C-d and C-u of copy mode, we use C-d and C-u to do page down and up instead of using prefix + C-d or prefix + C-u. That's the same reason.

About the implementation, it's flexible. As you said, it could be optional. I chose the easy way as I didn't think too much. Please change it to whatever you want.

[ddzero2c]

Related to #24