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...

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