It's impossible to find what fonts and fontstyles are available to After Effects via scripting. In Photoshop you simply use the app.fonts property to list all available fonts, but this is not included in the After Effects app object.

See this discussion: https://forums.adobe.com/thread/2646910

You can call a shell command to find available fonts, which works on mac, but since AE uses the postscript name for fonts it doesn't work properly on Windows, where this property is not available from the system. It makes scripting text very difficult, if not impossible