(core) Draft version of AI assistant

Summary:
The feature is behind a flag GRIST_FORMULA_ASSISTANT (must be "true"). But can be enabled in the
developer console by invoking GRIST_FORMULA_ASSISTANT.set(true).

Keys can be overriden in the document settings page.

Test Plan: For now just a stub test that checks if this feature is disabled by default.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3815

app/client/ui/CodeHighlight.ts

@@ -1,4 +1,4 @@
-import {colors, vars} from 'app/client/ui2018/cssVars';
+import {colors, theme, vars} from 'app/client/ui2018/cssVars';
 import * as ace from 'brace';
 import {BindableValue, dom, DomElementArg, styled, subscribeElem} from 'grainjs';
 
@@ -71,3 +71,18 @@ const cssHighlightedCode = styled(cssCodeBlock, `
     text-overflow: ellipsis;
   }
 `);
+
+export const cssFieldFormula = styled(buildHighlightedCode, `
+  flex: auto;
+  cursor: pointer;
+  margin-top: 4px;
+  padding-left: 24px;
+  --icon-color: ${theme.accentIcon};
+
+  &-disabled-icon.formula_field_sidepane::before {
+    --icon-color: ${theme.lightText};
+  }
+  &-disabled {
+    pointer-events: none;
+  }
+`);