feat(option): add option to close existing Jaq buffers

[naoki-mizuno]

Hi, in this PR I added an option to close existing Jaq buffers upon starting a new quickrun.

I like having a terminal quickrun buffer side-by-side with my source code buffer, and leave that open as I make modifications to the source code and rerun (see image at the bottom). However, every time I run :Jaq it opens a new quickrun buffer. By adding this option, it becomes possible to automatically close the buffer before running a new quickrun.

It seems that nvim_buf_get_option has been deprecated in 0.10, but I'm deliberately using it in order to make it consistent with the other uses of nvim_buf_{set,get}_option.

-- Note: Irrelevant options are omitted
require("jaq-nvim").setup({
  behavior = {
    default = "terminal",
    -- New option added in this PR
    -- closeexisting = true,
  },

  ui = {
    terminal = {
      position = "vert",
      size = 80,
      line_no = false
    },
  }
})

How I normally use jaq: show the results side-by-side, and edit, run, and repeat