rework crypto.ts

[ljoy913]

Updated crypto for Node and browser use:

• When running in Node >= 15, Node's web crypto api will be used for signature verification.
• When running in Node <= 14, Node's crypto lib will be used (web crypto is not available)
• When running in Browser, the built-in web crypto api will be used.
• When running in a browser without the web crypto api, a web crypto polyfill will be used.