icons [nfc]: Put directional-icons data in its own JSON file

Author: chrisbobbe

assets/icons/directional_icons.json

@@ -0,0 +1,5 @@
+[
+  "arrow_right",
+  "chevron_right",
+  "send"
+]

tools/icons/build-icon-font.js

@@ -62,12 +62,10 @@ async function main() {
   fs.copyFileSync(path.join(tmpDir, `${fontName}.ttf`), path.join(destDir, `${fontFileBaseName}.ttf`));
 
   const template = fs.readFileSync(dataTemplateFile, 'utf8');
-  // List of icons that should flip in RTL layouts
-  const directionalIcons = [
-    'arrow_right',
-    'chevron_right',
-    'send',
-  ];
+
+  // Icons that should flip horizontally in RTL layout.
+  const directionalIconsFile = path.join(srcDir, 'directional_icons.json');
+  const directionalIcons = fs.readFileSync(directionalIconsFile);
 
   const generated = Object.entries(codepoints).map(([name, codepoint]) => {
     const codepointHex = "0x" + codepoint.toString(16);