(core) Detect when a page initial is an emoji, avoid repeating it, and style it better

Summary:
- Detecting emoji is surprisingly tricky; we use a fancy regex as a decent heuristic.
- Icons are a little larger than before.
- Styling tweaked for light and dark modes
- In case the OS doesn't render the emoji as one character, truncate what's
  shown in the icon box.

Test Plan: Added a test case.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3904

app/common/ThemePrefs.ts

@@ -241,6 +241,8 @@ export interface ThemeColors {
   'left-panel-page-options-selected-hover-bg': string;
   'left-panel-page-initials-fg': string;
   'left-panel-page-initials-bg': string;
+  'left-panel-page-emoji-fg': string;
+  'left-panel-page-emoji-outline': string;
 
   /* Right Panel */
   'right-panel-tab-fg': string;

app/common/themes/GristDark.ts

@@ -220,6 +220,8 @@ export const GristDark: ThemeColors = {
   'left-panel-page-options-selected-hover-bg': '#A4A4A4',
   'left-panel-page-initials-fg': 'white',
   'left-panel-page-initials-bg': '#929299',
+  'left-panel-page-emoji-fg': 'black',
+  'left-panel-page-emoji-outline': '#69697D',
 
   /* Right Panel */
   'right-panel-tab-fg': '#EFEFEF',

app/common/themes/GristLight.ts

@@ -220,6 +220,8 @@ export const GristLight: ThemeColors = {
   'left-panel-page-options-selected-hover-bg': '#929299',
   'left-panel-page-initials-fg': 'white',
   'left-panel-page-initials-bg': '#929299',
+  'left-panel-page-emoji-fg': 'white',
+  'left-panel-page-emoji-outline': '#BDBDBD',
 
   /* Right Panel */
   'right-panel-tab-fg': '#262633',