Speed up strongRound by avoiding string casting (#1716)

I'm currently profiling my build pipeline and noticed that the
`strongRound` function showed up a couple of times, followed immediately
with a bit of GC cleanup shortly after. We can speed up that function by
avoiding the string casting caused by `Number.prototype.toFixed()`.

In my project's build pipeline with lots of svg icons this saves about
1.4s in total.

<img width="792" alt="js-tools-strongRound"
src="https://user-images.githubusercontent.com/1062408/204393563-666be3e0-e0ee-4608-9b7d-9ea8352bd36b.png">

plugins/convertPathData.js

@@ -951,6 +951,16 @@ function getIntersection(coords) {
   }
 }
 
+/**
+ * Does the same as `Number.prototype.toFixed` but without casting
+ * the return value to a string.
+ * @type {(num: number, precision: number) => number}
+ */
+function toFixed(num, precision) {
+  const pow = 10 ** precision;
+  return Math.round(num * pow) / pow;
+}
+
 /**
  * Decrease accuracy of floating-point numbers
  * in path data keeping a specified number of decimals.
@@ -960,16 +970,14 @@ function getIntersection(coords) {
  * @type {(data: number[]) => number[]}
  */
 function strongRound(data) {
-  for (var i = data.length; i-- > 0; ) {
-    // @ts-ignore
-    if (data[i].toFixed(precision) != data[i]) {
-      // @ts-ignore
-      var rounded = +data[i].toFixed(precision - 1);
+  const precisionNum = precision || 0;
+  for (let i = data.length; i-- > 0; ) {
+    const fixed = toFixed(data[i], precisionNum);
+    if (fixed !== data[i]) {
+      const rounded = toFixed(data[i], precisionNum - 1);
       data[i] =
-        // @ts-ignore
-        +Math.abs(rounded - data[i]).toFixed(precision + 1) >= error
-          ? // @ts-ignore
-            +data[i].toFixed(precision)
+        toFixed(Math.abs(rounded - data[i]), precisionNum + 1) >= error
+          ? fixed
           : rounded;
     }
   }