Added `{folder_id}` variable and parameter for subfolder support in `exp:file:entries` tag

[intoeetive]

Code: https://github.com/ExpressionEngine/ExpressionEngine/pull/3823

[jcogs-design]

In use {folder_id} returns the ID of the folder, which is confusingly referred to by directory_id in the CP URL when viewing the folder. So for this CP URL: /cp/files/directory/20&directory_id=434 {directory_id} returns 20 {folder_id} returns 434

In my use case this structure to successfully return the content of a sub-folder: {exp:file:entries folder_id="{folder_id}"}

HTH 🐾

[intoeetive]

@TomJaeger folder and subfolder are really synonymes. Feel free to change if that does not sound clear