Allow Browser Usage, countTokens convenience method

[wesmcouch]

Overview

• Remove fs and path usage to allow usage of the encoder and decoder both inside a browser and in nodejs
• Add a convenience method named countTokens that counts the number of tokens in a given string
• Update the example and tests to include the new method countTokens

[nem035]

FYI - I published a version to use (works in browser/node but no countTokens) until the maintainers merged: https://www.npmjs.com/package/@nem035/gpt-3-encoder

[arcticfly]

@nem035 looks like you can merge?

[astoilkov]

This PR also makes it work in Electron (not just the browser). Vite also has a problem with the code in master because it uses __dirname.