Add svg linter to check center

See simple-icons#3107 (comment)

.svglintrc.js

@@ -80,6 +80,17 @@ module.exports = {
               reporter.error("Unexpected character(s) detected outside the opening and/or closing <svg> tags");
             }
           },
+          function(reporter, $, ast) {
+            reporter.name = "icon-centered";
+            const iconPath = $.find("path").attr("d");
+            const bounds = getBounds(iconPath);
+
+            const centerX = +((bounds.minX + bounds.maxX) / 2).toFixed(iconFloatPrecision)
+            const centerY = +((bounds.minY + bounds.maxY) / 2).toFixed(iconFloatPrecision)
+            if (centerX !== iconSize / 2 && centerY !== iconSize / 2) {
+              reporter.error(`<path> must be centered on the canvas; the center is currently ${centerX} x ${centerY}`);
+            }
+          }
         ]
     }
 };