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

test/stub/render.ts

@@ -1,7 +1,7 @@
 import Liquid from '../../src/liquid'
 import { expect } from 'chai'
 
-const liquid = new Liquid()
+export const liquid = new Liquid()
 
 export const ctx = {
   date: new Date(),