Allow Save on newly created Sketch when in "external editor" mode

Please allow Save on newly created Sketch. This is helpful especially when the "Use external editor" is enabled.

Steps:
1. Enable "Use external editor" under Preferences
2. Create new sketch
3. Try to save

Reproduce the issue by enabling "Use external editor" under Preferences, creating a new sketch, and trying to save it. Trace the new-sketch save flow and the external-editor preference handling; the issue names no files or tests, so locate the relevant entry point first. Done means Save is available and successfully saves a newly created sketch in external-editor mode.

Written by the indexing model from the issue text.