Add support for legacy decorators when enabled in tsconfig/jsconfig

[timneutkens]

Unfortunately it seems that a group of apps is using legacy decorators in babelrc for integration with older mobx applications. We can add support for this through the experimentalDecorators flag in tsconfig/jsconfig as per @kdy1 that flag is the same as legacy decorators.

[balazsorban44]

This issue has been automatically locked due to no recent activity. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.