Add option scrollOnKeypress

Note that we also change the existing behavior a bit:
CoreService.triggerDataEvent() will not scroll to the bottom unless
`wasUserInput` is also true. This actually seems to be the intended
behavior according to the doc just above
ICoreService.triggerDataEvent().

src/browser/Terminal.ts

@@ -995,7 +995,7 @@ export class Terminal extends CoreTerminal implements ITerminal {
     const shouldIgnoreComposition = this.browser.isMac && this.options.macOptionIsMeta && event.altKey;
 
     if (!shouldIgnoreComposition && !this._compositionHelper!.keydown(event)) {
-      if (this.buffer.ybase !== this.buffer.ydisp) {
+      if (this.options.scrollOnKeypress && this.buffer.ybase !== this.buffer.ydisp) {
         this._bufferService.scrollToBottom();
       }
       return false;