Revert "provider: delete vimL stderr collector, now that it exists builtin"

This change exposed a memory issue with buffered channels, possibly
involving GC. Revert until it has been fixed.

This reverts commit 0de019b6a6.

runtime/autoload/provider.vim

@@ -0,0 +1,20 @@
+" Common functionality for providers
+
+let s:stderr = {}
+
+function! provider#stderr_collector(chan_id, data, event)
+   let stderr = get(s:stderr, a:chan_id, [''])
+   let stderr[-1] .= a:data[0]
+   call extend(stderr, a:data[1:])
+   let s:stderr[a:chan_id] = stderr
+endfunction
+
+function! provider#clear_stderr(chan_id)
+  if has_key(s:stderr, a:chan_id)
+    call remove(s:stderr, a:chan_id)
+  endif
+endfunction
+
+function! provider#get_stderr(chan_id)
+   return get(s:stderr, a:chan_id, [])
+endfunction