Installation
First, you need to install Tampermonkey extension or something similar. Then, click the "download" button. Tampermonkey should ask you whether you want to install the script. Install it (if you want, of course).
Then, you need to go to Tampermonkey settings, click the script name
and open the "Settings" tab. There you'll see a "Settings" section and
a "Run at" option. Choose "document-start" there, it should save
automatically. That's it, now the extension should activate itself
when you open the Glitch editor!
Don't forget the last step, it is important! Because Glitch editor
uses inline Content-Security-Policy, Tampermonkey cannot execute any
external scripts on its pages. Tampermonkey for now has no code to
override CSP in meta tags, only in HTTP headers. So we have to run
the code before your browser loads that Content-Security-Policy!
Hence "document-start".