don't rely on Array#flat

Former-commit-id: 66e713d422b34c8635b4e914d201944059ec0de0

lib/single-file/modules/css-fonts-minifier.js

@@ -77,7 +77,7 @@ this.singlefile.lib.modules.fontsMinifier = this.singlefile.lib.modules.fontsMin
 					}
 					return familyName;
 				}));
-				fontsInfo.used = fontsInfo.used.map(fontNames => fontNames.flat());
+				fontsInfo.used = fontsInfo.used.map(fontNames => flatten(fontNames));
 			}
 			const variableFound = fontsInfo.used.find(fontNames => fontNames.find(fontName => fontName.startsWith("var(--")));
 			let unusedFonts, filteredUsedFonts;
@@ -355,4 +355,8 @@ this.singlefile.lib.modules.fontsMinifier = this.singlefile.lib.modules.fontsMin
 		return FONT_WEIGHTS[weight.toLowerCase()] || weight;
 	}
 
+	function flatten(array) {
+		return array.reduce((a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []);
+	}
+
 })();