Add TypeScript autoimport support for deoplete (#2779)

* Add autoimport support for deoplete

* Fix test_deoplete_source.py

* Use callback instead of is_async for deoplete

Shuogo, the author of Deoplete, does not recommend using the `is_async`
option:

> I think is_async is not recommended. It is not so useful and broken.
> You should use callback system instead.

Link: https://github.com/Shougo/deoplete.nvim/issues/1006#issuecomment-526797857

Incidentally, the same thread mentiones an issue started by w0rp:
https://github.com/Shougo/deoplete.nvim/issues/976

The deoplete docs also say is_async is deprecated:

> is_async        (Bool)
>     If the gather is asynchronous, the source must set
>     it to "True". A typical strategy for an asynchronous
>     gather_candidates method to use this flag is to
>     set is_async flag to True while results are being
>     produced in the background (optionally, returning them
>     as they become ready). Once background processing
>     has completed, is_async flag should be set to False
>     indicating that this is the last portion of the
>     candidates.
>
>     Note: The feature is deprecated and not recommended.
>     You should use callback system by
>     |deoplete#auto_complete()| instead.

Link: https://github.com/Shougo/deoplete.nvim/blob/master/doc/deoplete.txt

Co-authored-by: w0rp <w0rp@users.noreply.github.com>

rplugin/python3/deoplete/sources/ale.py

@@ -24,6 +24,7 @@ class Source(Base):
         self.rank = 1000
         self.is_bytepos = True
         self.min_pattern_length = 1
+        self.is_volatile = True
         # Do not forget to update s:trigger_character_map in completion.vim in
         # updating entries in this map.
         self.input_patterns = {
@@ -44,21 +45,16 @@ class Source(Base):
         if not self.vim.call('ale#completion#CanProvideCompletions'):
             return None
 
-        if context.get('is_refresh'):
-            context['is_async'] = False
+        event = context.get('event')
 
-        if context['is_async']:
-            # Result is the same as for omnifunc, or None.
+        if event == 'Async':
             result = self.vim.call('ale#completion#GetCompletionResult')
+            return result or []
 
-            if result is not None:
-                context['is_async'] = False
-
-                return result
-        else:
-            context['is_async'] = True
-
-            # Request some completion results.
-            self.vim.call('ale#completion#GetCompletions', 'deoplete')
+        if context.get('is_refresh'):
+            self.vim.command(
+                "call ale#completion#GetCompletions('ale-callback', " + \
+                "{'callback': {completions -> deoplete#auto_complete() }})"
+            )
 
         return []