fix: request thread stack trace if client hasn't reported it (#395)

Request thread stack trace in case Debug Adapter hasn't reported it.
This makes it possible to see stack traces for every thread when
debugging multi-threaded debugee.
Fixes regression caused by:
a62e86b
Closes issue: #387

Author: sleeptightAnsiC

lua/dapui/components/frames.lua

@@ -25,6 +25,10 @@ return function(client, send_ready)
       end
 
       local frames = threads[thread_id].frames
+      if not frames then
+        local success, response = pcall(client.request.stackTrace, { threadId = thread_id })
+        frames = success and response.stackFrames
+      end
       if not frames then
         return
       end