Fix error emitting when double-clicking on JSON files in FileSystem tab

[DrewV3]

Fixes error thrown when clicking on a json file in the file dock

Bugsquad edit: Fix #66820

[Paulb23]

This change will no longer open scenes using the filedock if .tscn exists in the list of textfile_extensions. We have to give priority to the ResourceLoader.

[DrewV3]

Is that not the desired effect of adding .tscn to the list of textfile_extensions? As I understood it, the list of textfile extensions existed for the purpose of causing those files to be editable in the script editor instead of being imported. That's why none of the default items in textfile_extensions are resource types.

[Chaosus]

Please squash the commits as required by our pipeline and amend the commit name to be correct (we don't accept names like "fixes [link]").

[Paulb23]

textfile_extensions do not prevent the Resource from being imported. They where designed as a catch all for editing (text) files that are not imported (non-resources), but are still needed as part of development.

There is nothing stopping users from adding .tscn to the list and nor should we prevent it, as the user may have an engine plugin to edit other filetypes. We cannot not know this, hence we need to give priority to the ResourceLoader. As the user shouldn't have to edit their settings every time they open different projects.

Opening items in the filesystem dock should always take you to the dedicated editor for that Resource, regardless of what's in textfile_extensions. As the dedicated editor will be specialised to edit the Resource, as such should be the preferred workflow.

However, there is no prevention stopping the user from opening up a .tscn file if they wish, directly via ScriptEditor menus which bypasses textfile_extensions checks. Though not recommended.

[akien-mga]

Superseded by #72259.