chore(TypeScript): refactor objects into classes

fix: `Nil`(null, undefined) now renders as empty string
change: `parser.parseValue()` renamed to `parser.parseOutput`
change: registered tags/filters become static and shared across different liquid instances

test/.eslintrc.json

@@ -1,11 +0,0 @@
-{
-  "rules": {
-    "no-unused-expressions": "off"
-  },
-  "env": {
-    "mocha": true
-  },
-  "plugins": [
-    "mocha"
-  ]
-}