Add plugin types (#1527)

Covered following plugins
- addAttributesToSVGElement.js 
- addClassesToSVGElement.js 
- cleanupAttrs.js 
- convertEllipseToCircle.js 
- removeAttributesBySelector.js 
- removeAttrs.js 
- removeComments.js 
- removeDesc.js 
- removeDoctype.js 
- removeElementsByAttr.js 
- removeEmptyText.js 
- removeMetadata.js 
- removeRasterImages.js 
- removeScriptElement.js 
- removeStyleElement.js 
- removeTitle.js 
- removeXMLProcInst.js

plugins/convertEllipseToCircle.js

@@ -11,14 +11,16 @@ exports.description = 'converts non-eccentric <ellipse>s to <circle>s';
  * @see https://www.w3.org/TR/SVG11/shapes.html
  *
  * @author Taylor Hunt
+ *
+ * @type {import('../lib/types').Plugin<void>}
  */
 exports.fn = () => {
   return {
     element: {
       enter: (node) => {
         if (node.name === 'ellipse') {
-          const rx = node.attributes.rx || 0;
-          const ry = node.attributes.ry || 0;
+          const rx = node.attributes.rx || '0';
+          const ry = node.attributes.ry || '0';
           if (
             rx === ry ||
             rx === 'auto' ||