Make the UI more clear when you are editing a template part

[kellychoffman]

When you have a template part selected, it references the template and shows the template name in the dropdown and in the sidebar. There's no reference of the template part you are editing outside of chip the top bar.

[jameskoster]

We've tried a number of approaches here:

• A dotted outline on the ancestral template part container. (Removed in Try: Remove dotted ancestor border. #35637)
• A spotlight-esque effect that dims blocks outside the template part. (Still active)
• Displaying the template part name in the top bar. (Still active)
• A combination of all of these :)

At this point I would like to try making it so that template parts are only editable in focus mode. This seems the most bullet-proof way to clarify when you're editing a template part.

edit-in-isolation.mp4

[jameskoster]

Related: #35079

[mtias]

@noisysocks It doesn't seem we have a great direction here yet, let's punt from 5.9