[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/numpy-array/main.py

@@ -0,0 +1,16 @@
+# This one is tricky because `array` is an exported
+# symbol in a whole bunch of numpy internal modules.
+#
+# At time of writing (2025-10-07), the right completion
+# doesn't actually show up at all in the suggestions
+# returned. In fact, nothing from the top-level `numpy`
+# module shows up.
+arra<CURSOR: numpy.array>
+
+import numpy as np
+# In contrast to above, this *does* include the correct
+# completion. So there is likely some kind of bug in our
+# symbol discovery code for auto-import that isn't present
+# when using ty to discover symbols (which is likely far
+# too expensive to use across all dependencies).
+np.arra<CURSOR: array>