[javascript] Remove Buffer usage

[arjunyel]

Motivation

Buffer is Node only and gets converted to a string anyway so user should be responsible to convert based on their platform

Solution

Remove Buffer

[tasn]

The problem is that this breaks API unfortunately. :(

Is there a real problem with having it be there?

[arjunyel]

Buffer is a Node only thing, not standard JavaScript so it won't be acceptable in all runtimes.

@tasn would you prefer I close all my PRs and open them against the standard-webhooks library instead? That's much more new and I could do a 2.0 version there with breaking changes that won't effect as many people. In general though the Svix library has node only things which affects people like me who want to use it in other environments like Cloudflare workers, we have to use polyfills