fix settings editor memory leak

[SimonSiefke]

Helps with #216649

Stack Trace

By measuring growing disposable stores, it showed this stack trace very often:

SettingsEditor2._register (vscode/out/vs/base/common/lifecycle.js:393:32)\nvscode/out/vs/workbench/contrib/preferences/browser/settingsEditor2.js:264:22

// settingsEditor2.js
this._register(input.onWillDispose(() => {
	this.searchWidget.setValue('');
}));

Possible memory leak cause

It seems when all editors are closed, the last opened editor always remains. But when setting a new editor input, a input.onWillDispose listener is added to the SettingsEditor2. When opening the SettingsEditor2 again, a new input.onWillDispose listener is added to the initial SettingsEditor2.

Testing

For testing, I ran the same test script opening and closing the settings editor 197 times which showed that the number of SettingsEditor2Input instances was reduced from 197 to 2.