Fix browse_dialog in Blender scene importer to accept files

[raging-loon]

Changes EditorFileSystemImportFormatSupportQueryBlend::browse_dialog to accept file names. As it is now, it only accepts directory names. This will present an error as the callback "_select_install" and subsequent calls to "_validate_path" and "_get_blender_version" are all expecting filenames.

Closes #93235

[Calinou]

Tested locally, it works as expected.

However, the UI still refers to a Blender installation, which can be a bit ambiguous. I suggest renaming it to Blender executable instead.

On macOS, it might be worth hinting that you should point at Contents/MacOS/blender within Blender.app too (e.g. by appending to the label with #ifdef MACOS_ENABLED).

[raging-loon]

I could change "installation" to "executable". However, for Mac support, it would be better for someone else handle that, as I have never touched a Mac let along program one.

[akien-mga]

I pushed an update myself that adjusts the label and error messages for clarity.

[akien-mga]

For the record, I noticed #94914 while working on this, but it doesn't seem related to these changes, the bug also happens in RC 1.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉