[ty] Add an evaluation for completions

This is still early days, but I hope the framework introduced here makes
it very easy to add new truth data. Truth data should be seen as a form
of regression test for non-ideal ranking of completion suggestions.

I think it would help to read `crates/ty_completion_eval/README.md`
first to get an idea of what you're reviewing.

crates/ty_completion_eval/truth/object-attr-instance-methods/main.py

@@ -0,0 +1,16 @@
+class Quux:
+    def __init__(self): pass
+    def lion(self): pass
+    def tiger(self): pass
+    def bear(self): pass
+    def chicken(self): pass
+    def turkey(self): pass
+    def wasp(self): pass
+    def rabbit(self): pass
+    def squirrel(self): pass
+
+quux = Quux()
+quux.tur<CURSOR: turkey>
+
+quux = Quux()
+quux.be<CURSOR: bear>