Document @export_tool_button icon parameter in GDScript exports

[Calinou]

• See Add ability to choose custom icon for @export_tool_button godot-proposals#12124 (comment).

I've started working on a website to browse editor icons, which should make it easier to choose an icon compared to the file list provided by GitHub. (This could also be made as a documentation page that's generated with a script, if we want to have this directly in the documentation.)

Edit: The website in question is up, I've linked it in the PR: https://godot-editor-icons.github.io

[MarianoGnu]

I've started working on a website to browse editor icons, which should make it easier to choose an icon compared to the file list provided by GitHub.

I wonder how hard would be to add autocompletion with previsualization of icons to the second parameters, we already have specialized code for signals in emit_signal() and input events in Input.is_action_pressed() and similar.

However, from my little experience working with GdScript source code, annotations are tricky because they are parsed before the rest of the sources in an earlier stage 🤔