Fix 'The operand of a 'delete' operator must be optional' errors in VS Code codebase

[mjbvz]

The latest TS 3.9 nightly catches a potential strict null check violation where you delete a property that is not marked optional:

interface Foo {
   x: number;
}

const a: Foo = { x: 3 };

delete a.x // new error here since `x` is not marked as being potentially undefined 

This revealed a few potential errors in our code:

• vs/workbench/contrib/tasks/browser/taskQuickPick.ts(56,11) @alexr00
• vs/workbench/contrib/tasks/browser/taskQuickPick.ts(57,11) @alexr00
• vs/workbench/contrib/tasks/common/taskConfiguration.ts(1204,13) @alexr00
• vs/workbench/contrib/search/test/browser/queryBuilder.test.ts(954,10) @roblourens
• vs/workbench/contrib/search/test/browser/queryBuilder.test.ts(955,10) @roblourens
• vs/workbench/contrib/search/common/searchModel.ts(1103,10) @roblourens
• vs/workbench/contrib/debug/browser/debugToolBar.ts(274,11) @isidorn
• vs/workbench/browser/parts/statusbar/statusbarPart.ts(706,12) @bpasero
• vs/platform/environment/node/argv.ts(253,9) @aeschli
• vs/workbench/api/node/extHostDebugService.ts(100,12) @weinand
• vs/code/electron-main/window.ts(762,11) @bpasero
• vs/code/electron-main/app.ts(196,12) @mjbvz
• vs/server/remoteCli.ts(128,9) @aeschli

I've made some initial assignments but please reassign if you are not the correct owner

[mjbvz]

I've marked this as April since it blocks us from picking up the latest TS build and providing feedback on it

/cc @DanielRosenwasser for awareness of the impact of this change

[mjbvz]

Actually looks like TS may be reverting this change for TS 3.9. We should still fix these issues but don't need to do it this iteration

[alexr00]

@mjbvz How can force the error so I can make sure it's fixed?

[mjbvz]

In the root of the VS Code project:

• yarn add typescript@3.9.0-dev.20200423
• Then restart the watch task and you should start seeing the errors

[mjbvz]

Closing this since most bugs have been fixed. I believe that TS 4.0 should add this check back