(core) Conditional formatting rules

Summary:
Adding conditional formatting rules feature.

Each column can have multiple styling rules which are applied in order
when evaluated to a truthy value.

- The creator panel has a new section: Cell Style
- New user action AddEmptyRule for adding an empty rule
- New columns in _grist_Table_columns and fields

A new color picker will be introduced in a follow-up diff (as it is also
used in choice/choice list/filters).

Design document:
https://grist.quip.com/FVzfAgoO5xOF/Conditional-Formatting-Implementation-Design

Test Plan: new tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3282

app/client/declarations.d.ts

@@ -303,6 +303,7 @@ declare module "app/client/models/DataTableModel" {
 declare module "app/client/lib/koUtil" {
   export interface ComputedWithKoUtils<T> extends ko.Computed<T> {
     onlyNotifyUnequal(): this;
+    previousOnUndefined(): this;
   }
   export interface ObservableWithKoUtils<T> extends ko.Observable<T> {
     assign(value: unknown): this;