Add setting to disable theme switching in vscode

[austinabell]

Is your feature request related to a problem? Please describe.
Couldn't find the repo for the extension if it's open source, but it's distracting that the leo vscode extension overrides the color theme of the editor when opening and closing Leo files.

Describe the solution you'd like
A setting tied to the extension to turn this off.

Describe alternatives you've considered

Additional context