feat: channel and multi-concurrent task join support for the plugin system

[sxyazi]

Two new APIs added:

• ya.chan() - Create a channel
• ya.join() - Join multi-concurrent tasks

For example:

local keys = {
  { on = "q", run = "quit" },
  { on = "k", run = "up" },
  { on = "j", run = "down" },
  { on = "h", run = "left" },
  { on = "l", run = "right" },
}

local tx, rx = ya.chan("mpsc")

function producer()
  while true do
    local cand = self.keys[ya.which { cands = self.keys, silent = true }]
    if cand then
      tx:send(cand.run)
      if cand.run == "quit" then
        break
      end
    end
  end
end

function consumer()
  repeat
    local run = rx:recv()
    if run == "quit" then
      break
    elseif run == "up" then
      ya.dbg("up")
    elseif run == "down" then
      ya.dbg("down")
    elseif run == "left" then
      ya.dbg("left")
    elseif run == "right" then
      ya.dbg("right")
    end
  until not run
end

ya.join(producer, consumer)