Changing from global to per-editor should explicitly set the overtype mode for all open editors

[AdamMaras]

When the extension configuration is changed at runtime and per-editor mode is enabled, all the currently open editors should have their insert/overtype mode set to whatever the global mode was set to when the configuration was changed.

The "easy" fix is currently blocked by Microsoft/vscode#15178 since there's not currently a Visual Studio Code extension API for enumerating all open editors. There may be a better/cleaner way to go about this that I haven't thought about—if anyone has an idea, I'm all ears.

For reference in code: mode.ts@1c73719 line 43