(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/models/Styles.ts

@@ -0,0 +1,19 @@
+export interface Style {
+  textColor?: string;
+  fillColor?: string;
+}
+
+export class CombinedStyle implements Style {
+  public readonly textColor?: string;
+  public readonly fillColor?: string;
+  constructor(rules: Style[], flags: any[]) {
+    for (let i = 0; i < rules.length; i++) {
+      if (flags[i]) {
+        const textColor = rules[i].textColor;
+        const fillColor = rules[i].fillColor;
+        this.textColor = textColor || this.textColor;
+        this.fillColor = fillColor || this.fillColor;
+      }
+    }
+  }
+}