Towards `useDefineForClassFields` (#248695)

* final init order fixes https://github.com/microsoft/vscode/issues/243049

* a few more fixed when compiling with `"useDefineForClassFields": true`

https://github.com/microsoft/vscode/issues/243049 and https://github.com/microsoft/vscode/issues/186726

* add a dry-check that ensures we can/could compile/emit with explicitly defined class fields

https://github.com/microsoft/vscode/issues/186726