[javascript] Remove fetch polyfill

[arjunyel]

Motivation

svix-fetch uses Node specific APIs which causes issues when using non-node environments, like Cloudflare Workers, and was imported as a side effect which means that bundlers cannot tree shake svix. fetch is now supported on all maintained Nodejs versions. Also as a general rule users should be responsible for polyfilling fetch, not libraries.

Solution

Use fetch from globalThis and mark the library as side effect free.

[svix-lucho]

@arjunyel The reason why we use svix-fetch is not to polyfill fetch, it's to enable keepalives when running in a node context. I agree with the issues with non-node environments and tree-shaking, but we need to make sure that we are not losing the keepalive behavior when needed.

UPDATE: keepalive is enabled by default as of Node 19 - https://nodejs.org/en/blog/announcements/v19-release-announce#https11-keepalive-by-default. So I think we can go ahead with this for future versions.

Discussed this with the team, and we decided that Node 19 is just too recent to depend on it.

[arjunyel]

The end user can set keep alive on their fetch for Node 18 https://stackoverflow.com/questions/62500011/reuse-tcp-connection-with-node-fetch-in-node-js