Add support for configuring clipboard used for mouse yank

[apanloco]

Adds mouse_yank_clipboard configuration option under the editor section:

[editor] mouse-yank-clipboard = "system" / "primary" (default "primary")

This affects the following actions:

• Selecting text with mouse will yank to configured clipboard
• Pasting with middle mouse button will use configured clipboard
• Replacing selected text with ALT+middle mouse button will use configured clipboard

Without this option Helix will fallback to current behavior of using the primary clipboard for the above mentioned actions.

Fixes #6642.

[Dreamacro]

Very useful feature, hope it can be merged 🙏

[apanloco]

Rebased, and made sure it still works. The wording of Primary and System might not be very relevant any longer, since the message is now "yanked ... to register * / +". Not sure though.

[the-mikedavis]

Instead we could deserialize to a char and yank to the register at that char. So you could choose between +/* but also regular registers like " (the default yank register) or other special registers like _ (black hole - the yank would be discarded).

So this would be mouse-yank-register instead.