(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/text.py

@@ -5,6 +5,7 @@ import dateutil.parser
 import numbers
 import re
 
+from unimplemented import unimplemented
 from usertypes import AltText   # pylint: disable=import-error
 
 def CHAR(table_number):
@@ -499,6 +500,7 @@ def T(value):
           str(value) if isinstance(value, AltText) else "")
 
 
+@unimplemented
 def TEXT(number, format_type):
   """
   Converts a number into text according to a specified format. It is not yet implemented in