(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/lookup.py

@@ -1,57 +1,72 @@
 # pylint: disable=redefined-builtin, line-too-long
+from unimplemented import unimplemented
 
+@unimplemented
 def ADDRESS(row, column, absolute_relative_mode, use_a1_notation, sheet):
   """Returns a cell reference as a string."""
   raise NotImplementedError()
 
+@unimplemented
 def CHOOSE(index, choice1, choice2):
   """Returns an element from a list of choices based on index."""
   raise NotImplementedError()
 
+@unimplemented
 def COLUMN(cell_reference=None):
   """Returns the column number of a specified cell, with `A=1`."""
   raise NotImplementedError()
 
+@unimplemented
 def COLUMNS(range):
   """Returns the number of columns in a specified array or range."""
   raise NotImplementedError()
 
+@unimplemented
 def GETPIVOTDATA(value_name, any_pivot_table_cell, original_column_1, pivot_item_1=None, *args):
   """Extracts an aggregated value from a pivot table that corresponds to the specified row and column headings."""
   raise NotImplementedError()
 
+@unimplemented
 def HLOOKUP(search_key, range, index, is_sorted):
   """Horizontal lookup. Searches across the first row of a range for a key and returns the value of a specified cell in the column found."""
   raise NotImplementedError()
 
+@unimplemented
 def HYPERLINK(url, link_label):
   """Creates a hyperlink inside a cell."""
   raise NotImplementedError()
 
+@unimplemented
 def INDEX(reference, row, column):
   """Returns the content of a cell, specified by row and column offset."""
   raise NotImplementedError()
 
+@unimplemented
 def INDIRECT(cell_reference_as_string):
   """Returns a cell reference specified by a string."""
   raise NotImplementedError()
 
+@unimplemented
 def LOOKUP(search_key, search_range_or_search_result_array, result_range=None):
   """Looks through a row or column for a key and returns the value of the cell in a result range located in the same position as the search row or column."""
   raise NotImplementedError()
 
+@unimplemented
 def MATCH(search_key, range, search_type):
   """Returns the relative position of an item in a range that matches a specified value."""
   raise NotImplementedError()
 
+@unimplemented
 def OFFSET(cell_reference, offset_rows, offset_columns, height, width):
   """Returns a range reference shifted a specified number of rows and columns from a starting cell reference."""
   raise NotImplementedError()
 
+@unimplemented
 def ROW(cell_reference):
   """Returns the row number of a specified cell."""
   raise NotImplementedError()
 
+@unimplemented
 def ROWS(range):
   """Returns the number of rows in a specified array or range."""
   raise NotImplementedError()