Python: Fix some more async parsing problems

Turns out we were not setting the `is_async` field on anything except
`async for` statements. This commit makes it so that we also do this for
`async def` and `async with`, and adds a test that this produces the
same behaviour as the old parser.

python/extractor/tests/parser/async-await.py

@@ -0,0 +1,8 @@
+async def foo():
+    await bar() + await baz()
+
+    async with foo() as bar, baz() as quux:
+        pass
+
+    async for spam in eggs:
+        pass

python/extractor/tsg-python/python.tsg

@@ -2650,6 +2650,14 @@
     let @with.first = @first.node
 }
 
+; Async status
+; NOTE: We only set the `is_async` field on the _first_ clause of the `with` statement,
+; as this is the behaviour of the old parser.
+(with_statement "async" "with" @with_keyword (with_clause . (with_item) @with))
+{
+    attr (@with.node) is_async = #true
+}
+
 (with_item
     value: (_) @value
 ) @with
@@ -3253,6 +3261,16 @@
     }
 }
 
+; Async status
+(function_definition "async" "def" @def_keyword) @funcdef
+{
+    let start = (location-start @def_keyword)
+    attr (@funcdef.function) is_async = #true
+    attr (@funcdef.node) _location_start = start
+    attr (@funcdef.function) _location_start = start
+    attr (@funcdef.funcexpr) _location_start = start
+}
+
 ;;; Decorators
 
 (decorated_definition