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/addAttributesToSVGElement.js

@@ -48,11 +48,16 @@ plugins: [
  * Add attributes to an outer <svg> element. Example config:
  *
  * @author April Arcus
+ *
+ * @type {import('../lib/types').Plugin<{
+ *   attribute?: string | Record<string, null | string>,
+ *   attributes?: Array<string | Record<string, null | string>>
+ * }>}
  */
 exports.fn = (root, params) => {
   if (!Array.isArray(params.attributes) && !params.attribute) {
     console.error(ENOCLS);
-    return;
+    return null;
   }
   const attributes = params.attributes || [params.attribute];
   return {
@@ -62,12 +67,14 @@ exports.fn = (root, params) => {
           for (const attribute of attributes) {
             if (typeof attribute === 'string') {
               if (node.attributes[attribute] == null) {
+                // @ts-ignore disallow explicit nullable attribute value
                 node.attributes[attribute] = undefined;
               }
             }
             if (typeof attribute === 'object') {
               for (const key of Object.keys(attribute)) {
                 if (node.attributes[key] == null) {
+                  // @ts-ignore disallow explicit nullable attribute value
                   node.attributes[key] = attribute[key];
                 }
               }