feat(run): add get_command function

[joe-p]

Adds a new run.get_command function that returns the command (string[]) that would be ran for the given args.

For example, I have added a keybind to yank the command for the nearest test:

    {
      '<leader>ty',
      function()
        local cmd = require('neotest').run.get_command()

        if cmd then
          vim.fn.setreg('*', table.concat(cmd, ' '))
          -- get_command may block for a bit, so send a notification when it is done
          vim.notify('Copied ' .. cmd[1] .. ' command!')
        end
      end,
      desc = 'Yank',
    },

Closes #272