Expose a documentOverride setting

This lets the embedder choose which Document object to use for creating
elements, adding event listeners, etc. The reason this exists is because
when working with multiple windows it can be convenient to create all
elements under the primary window to make instanceof usage consistent.

See microsoft/vscode#195595

src/browser/Terminal.ts

@@ -398,6 +398,9 @@ export class Terminal extends CoreTerminal implements ITerminal {
     }
 
     this._document = parent.ownerDocument!;
+    if (this.options.documentOverride && this.options.documentOverride instanceof Document) {
+      this._document = this.optionsService.rawOptions.documentOverride as Document;
+    }
 
     // Create main element container
     this.element = this._document.createElement('div');

src/common/services/OptionsService.ts

@@ -18,6 +18,7 @@ export const DEFAULT_OPTIONS: Readonly<Required<ITerminalOptions>> = {
   cursorInactiveStyle: 'outline',
   customGlyphs: true,
   drawBoldTextInBrightColors: true,
+  documentOverride: null,
   fastScrollModifier: 'alt',
   fastScrollSensitivity: 5,
   fontFamily: 'courier-new, courier, monospace',

src/common/services/Services.ts

@@ -217,6 +217,7 @@ export interface ITerminalOptions {
   cursorInactiveStyle?: CursorInactiveStyle;
   customGlyphs?: boolean;
   disableStdin?: boolean;
+  documentOverride?: any | null;
   drawBoldTextInBrightColors?: boolean;
   fastScrollModifier?: 'none' | 'alt' | 'ctrl' | 'shift';
   fastScrollSensitivity?: number;