Moved feDropShadow to the svg filter allowlist

The reason it was there was because "If I remember correctly there was some mXSS risk connected to those", however I searched and couldn't find one (and neither could cure53 #573 (comment)) and so I change it back.

src/tags.js

@@ -176,6 +176,7 @@ export const svgFilters = freeze([
   'feDiffuseLighting',
   'feDisplacementMap',
   'feDistantLight',
+  'feDropShadow',
   'feFlood',
   'feFuncA',
   'feFuncB',
@@ -203,7 +204,6 @@ export const svgDisallowed = freeze([
   'color-profile',
   'cursor',
   'discard',
-  'fedropshadow',
   'font-face',
   'font-face-format',
   'font-face-name',