Adjust Settings editor narrow total width

[rzhao271]

Fixes #145243
Fixes #143690

We increase the Settings editor narrow total width. In turn, the ToC only shows up when the Settings editor is wide enough.

Pros:

• When the ToC shows up, it doesn't show up ridiculously narrow.
• When the ToC shows up, the sash already has 100px of wiggle room.

Cons:

• The splitview total width must be at least 700px wide for the ToC to even show up. This is calculated based on the ToC reset width, which is 200px, as well as the editor side min width, which is currently at 500px. The editor side min width is that wide because we currently don't have a horizontal scroll bar for the editor side in case of min overflow, and because string setting inputboxes are currently very long, in case they represent path settings. "Editor side" means the non-ToC portion of the Settings editor, in this case.

Considering how users who use VS Code with a narrower width might get annoyed at the ToC not showing up, a better solution is probably to figure out how to add a horizontal scrollbar to just the editor side in case it is too narrow.

Adding @misolori as a reviewer for thoughts on this issue.
Also CC @stevencl.

[miguelsolorio]

LGTM, here are the screenshots of where the narrow view kicks in: