Align/cleanup `_serviceBrand` usage

[jrieken]

We have a convention of using the _serviceBrand-properties on arguments that get injected to constructors. This is to break structural typing and to make sure IInstantiationService#createInstance bubbles back those (non-service) types it needs to be called with.

The _serviceBrand-property is typed as any and different usages patterns have emerged - full text search for _serviceBrand!?: .*?(;|,).

This should be cleaned up like so

• make the type undefined
  • so that strict-init-tricks aren't needed, e.g. this _serviceBrand!: ServiceIdentifier<ILifecycleService>; and
  • so that _serviceBrand isn't assigned values (it's a compile time thing only)
• rename the property to _injectableBrand because this is about types being injected, not necessarily services.

[jrieken]

fyi @microsoft/vscode

[jrieken]

Un-assigning folks, I will just do it using find/replace magic