allow configuring the default register

[jyn514]

i like to share a clipboard across all my applications. helix allows me to select the register to use for the clipboard ("), including the system clipboard ("+), but it does not allow me to choose the default clipboard, so every yank breaks my muscle memory and requires 3x as many strokes. it would be nice to make this configurable with an option. currently it's hard-coded to ", e.g.

helix/helix-term/src/commands.rs

Line 2656 in cfe80ac

let reg_name = cx.register.unwrap_or('"');

[the-mikedavis]

Duplicate of #2038, see #2038 (comment) in particular. My comment there about multi selection yank and paste is out of date actually - that works now when yanking & pasting within the same Helix session.