fix: math filters now return number, resolves #110

* fix linting
* numbers are no longer fixed:
    i.e. {{0.3 | minus 0.1}} now renders as 0.19999
    Stick to JavaScript number behavior, see: 8.0.0

src/drop/drop.ts

@@ -1,7 +1,5 @@
-import { deprecate } from '../util/deprecate'
-
 export abstract class Drop {
-  valueOf(): any {
+  valueOf (): any {
     return undefined
   }