(core) Improve suggestions for formula autocomplete

Summary:
- Make suggestions less case-sensitive (not entirely case-insensitive, but
  allow top-level suggestions to match in all-lowercase)
- Add function signatures to suggestions for Grist functions.
- Excel-like functions that are present but not implemented are no longer
  offered as suggestions.

Test Plan:
Added a test case on python side, and a browser test case for how suggestions
are rendered and inserted.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2608

sandbox/grist/functions/unimplemented.py

@@ -0,0 +1,11 @@
+"""
+Decorator that marks functions as not implemented. It sets func.unimplemented=True.
+Usage:
+
+@unimplemented
+def func(...):
+  raise NotImplemented
+"""
+def unimplemented(func):
+  func.unimplemented = True
+  return func