And they even have a name: userscripts! [1]

Chrome also used to natively support userscripts back in 2010 [2] but they mostly killed it off

[1] https://en.wikipedia.org/wiki/Userscript

[2] https://lifehacker.com/chrome-4-supports-greasemonkey-usersc...

There's also Violentmonkey [1][2] (which is said to be more lightweight) and Tampermonkey.

Tampermonkey has analytics enabled, and is closed source.

Here's a comprehensive summary on all three. [3]

[1] https://addons.mozilla.org/en-US/firefox/addon/violentmonkey

[2] https://violentmonkey.github.io/

[3] https://news.ycombinator.com/item?id=26057106

It certainly is great to have first-party support for such a simple feature. It doesn't have to support the whole GM_ API