Use Object.create(null) to avoid inherit properties like constructor

[sileix]

Objects created from literal syntax will inherit properties from
Object.prototype, which includes constructor.
As a result, lexcon['constructor'] will return a function object
instead of undefined.

This bug was discovered while using en-pos, and can be reproduced by the following code:

const Tag = require("en-pos").Tag;
const tags = new Tag(["constructor"]).initial() .smooth().tags;

During initial(), it calls lexicon.ts where it assumes the return value of lexicon[token] is either undefined or string.