add information on valid file names for source code mirroring

[gpunktschmitz]

Describe the change you'd like: when trying to mirror my style from my github project I was unlucky adding my file gpunktstyle.css - searching the github issues of this project I eventually found the valid file names (endings on user.css and user.styl from question #198).

When entering an invalid file name it would be helpful to show the valid file names as hint/error message.

[astyled]

Well this double extension is Stylus requirement and probably part of UserCSS spec (if it exists), so I would assume users should know about it.

We can improve import URL placeholder at /import and add an error message to mirror URL placeholder like name/description length check uses.

[gpunktschmitz]

I was using stylus for years and did not hear/read about it until yesterday. you can create user styles in the browser and on userstyles.world without ever knowing about any extension. why would you not show an appropriate error message when the extension is wrong? or maybe add information on file extension to the faq? https://userstyles.world/docs/faq#how-does-mirroring-source-code-work
I for one had a hard time finding that out.

[astyled]

I agree we need to improve our errors and documentation.

However, I think the root cause here is Stylus implementing "the medium of userstyle" as "Stylus userstyle", not "userstyle file". There's a constant flow of people who don't know they could easily use any version control system (like git) and any text editor/IDE for developing their userstyles, as well as any code sharing platform for publishing it. If they knew it, their experience with userstyles could have been improved a lot.