(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/common/gristTypes.ts

@@ -341,3 +341,9 @@ export function isRefListType(type: string) {
 export function isFullReferencingType(type: string) {
   return type.startsWith('Ref:') || isRefListType(type);
 }
+
+export function isValidRuleValue(value: CellValue|undefined) {
+  // We want to strictly test if a value is boolean, when the value is 0 or 1 it might
+  // indicate other number in the future.
+  return value === null || typeof value === 'boolean';
+}

app/common/schema.ts

@@ -4,7 +4,7 @@ import { GristObjCode } from "app/plugin/GristData";
 
 // tslint:disable:object-literal-key-quotes
 
-export const SCHEMA_VERSION = 26;
+export const SCHEMA_VERSION = 27;
 
 export const schema = {
 
@@ -38,6 +38,7 @@ export const schema = {
     summarySourceCol    : "Ref:_grist_Tables_column",
     displayCol          : "Ref:_grist_Tables_column",
     visibleCol          : "Ref:_grist_Tables_column",
+    rules               : "RefList:_grist_Tables_column",
     recalcWhen          : "Int",
     recalcDeps          : "RefList:_grist_Tables_column",
   },
@@ -125,6 +126,7 @@ export const schema = {
     displayCol          : "Ref:_grist_Tables_column",
     visibleCol          : "Ref:_grist_Tables_column",
     filter              : "Text",
+    rules               : "RefList:_grist_Tables_column",
   },
 
   "_grist_Validations": {
@@ -226,6 +228,7 @@ export interface SchemaTypes {
     summarySourceCol: number;
     displayCol: number;
     visibleCol: number;
+    rules: [GristObjCode.List, ...number[]]|null;
     recalcWhen: number;
     recalcDeps: [GristObjCode.List, ...number[]]|null;
   };
@@ -313,6 +316,7 @@ export interface SchemaTypes {
     displayCol: number;
     visibleCol: number;
     filter: string;
+    rules: [GristObjCode.List, ...number[]]|null;
   };
 
   "_grist_Validations": {