chore: fix update-specs script, use builtin fetch

Description

Fixes the update specs script being broken, removes unnecessary dependency

In master, when you run update specs, it fails to update commonmark specs.

This is because the place it pulls from, https://spec.commonmark.org/<version>/spec.json seems like it doesn't accept versions numbers without a patch anymore, and the current script sends a request for major.minor. It also doesn't accept requests for major.minor.0, or, as far as I could tell, anything except the current version. Therefore, we can just use the version from commonmark's package.json instead of lowering it to major.minor.

This does mean the resulting spec file gets renamed, but I think having the full version is better if we're actually pulling from the full version.

The script was also using node-fetch, but fetch has existed natively in node since (I think) node 18 so it's within marked's support window.

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markedjs / marked