add "hello_nim" example application written by Nim.

Signed-off-by: Takeyoshi Kikuchi <kikuchi@centurysys.co.jp>

examples/hello_nim/hello_nim_async.nim

@@ -0,0 +1,18 @@
+import std/asyncdispatch
+import std/strformat
+
+proc task(id: int): Future[void] {.async.} =
+  for loop in 0..2:
+    echo &"Hello from task {id}! loops: {loop}"
+    if loop < 2:
+      await sleepAsync(1000)
+
+proc launch() {.async.} =
+  for id in 1..2:
+    asyncCheck task(id)
+    await sleepAsync(200)
+  await task(3)
+
+proc hello_nim() {.exportc, cdecl.} =
+  waitFor launch()
+  GC_runOrc()