(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/ui/RightPanel.ts

@@ -599,7 +599,7 @@ export const cssButtonRow = styled(cssRow, `
 
 export const cssIcon = styled(icon, `
   flex: 0 0 auto;
-  background-color: ${colors.slate};
+  --icon-color: ${colors.slate};
 `);
 
 const cssTopBarItem = styled('div', `