Document language extensions icons

[deribaucourt]

While developing language extensions, I had the need to add a custom icon for my language's files. microsoft/vscode#118846 implemented a way to do exactly that but it is not part of the documentation https://code.visualstudio.com/api/language-extensions/overview.

[lszomoru]

@jrieken, @mjbvz, could you help with routing this issue to the correct owner? Thanks!