Deprecate fast scroll modifier as it's no longer possible

Not possible via the component we import. We could add this but I
doubt it was being used anyway.

src/common/services/Services.ts

@@ -219,6 +219,7 @@ export interface ITerminalOptions {
   disableStdin?: boolean;
   documentOverride?: any | null;
   drawBoldTextInBrightColors?: boolean;
+  /** @deprecated No longer supported */
   fastScrollModifier?: 'none' | 'alt' | 'ctrl' | 'shift';
   fastScrollSensitivity?: number;
   fontSize?: number;