Add the buffer polyfill as a dependency

[Vinnl]

In browser-based environments, require("buffer") will fail unless
buffer is available as a dependency and can be provided by a
bundler. This used to automatically be the case in e.g. older
versions of Webpack, but since Webpack v5 no longer automatically
includes polyfills for Node built-ins, it can happen that the
buffer polyfill is no longer available in a project's dependency
tree.

Fixes #18 and nodejs/readable-stream#448.

[jimmywarting]

safe buffer is only needed for node < v6
you should be using Buffer directly instead. or better yet, use Uint8Array instead. it's better for cross compatibility