refactor(filter): Filters should be private, we only expose resolvable

Sources/Namespace.swift

@@ -1,4 +1,4 @@
-public protocol FilterType {
+protocol FilterType {
   func invoke(value: Any?, arguments: [Any?]) throws -> Any?
 }
 

Sources/Parser.swift

@@ -76,7 +76,7 @@ public class TokenParser {
     tokens.insert(token, at: 0)
   }
 
-  public func findFilter(_ name: String) throws -> FilterType {
+  func findFilter(_ name: String) throws -> FilterType {
     if let filter = namespace.filters[name] {
       return filter
     }