feat: make the yank register configurable

[mikesmithgh]

Currently, the register '+' copies text and immediately exits. This register is hardcoded in the autocommands and keymaps, make this configurable similar to the paste window register.

See

• https://github.com/mikesmithgh/kitty-scrollback.nvim/blob/4696f080f082377e9c2ce5f280abc1b02d33137b/lua/kitty-scrollback/autocommands.lua#L125
• https://github.com/mikesmithgh/kitty-scrollback.nvim/blob/main/lua/kitty-scrollback/keymaps.lua#L51

Using the example from https://github.com/mikesmithgh/kitty-scrollback.nvim/issues/152

        global = function()
          vim.keymap.set({ 'v' }, 'Y', '<Plug>(KsbVisualYankLine)', {})
          vim.keymap.set({ 'v' }, 'y', '<Plug>(KsbVisualYank)', {})
          vim.keymap.set({ 'n' }, 'Y', '<Plug>(KsbNormalYankEnd)', {})
          vim.keymap.set({ 'n' }, 'y', '<Plug>(KsbNormalYank)', {})
          vim.keymap.set({ 'n' }, 'yy', '<Plug>(KsbYankLine)', {})

          vim.keymap.set({ 'v' }, '<leader>Y', '"aY', {})
          vim.keymap.set({ 'v' }, '<leader>y', '"ay', {})
          vim.keymap.set({ 'n' }, '<leader>Y', '"aY', {})
          vim.keymap.set({ 'n' }, '<leader>y', '"ay', {})
          vim.keymap.set({ 'n' }, '<leader>yy', '"ayy', {})
          return {
            paste_window = { yank_register = 'a' },
          }

yanks to other registers, for example b still go to the register +.

The current state makes it difficult to modify the behavior of yanks opening the paste window vs normal behavior vs copying to the clipboard and quitting.