Read jsConfig baseUrl if its present, and use it as a loader base for modules

In order to properly support the existing specifier paths used for load in the mongo test runner, we need to either:

• assume we are always running the test runner from the same directory
• use some standard convention for determining the base URL for running tests

The code for load assumes the first, which is slightly problematic since you can't run jstests from any folder. VSCode/TypeScript use jsconfig.json to support a baseUrl option, which informs module loaders what the prefix for specifiers should be. This ticket will cover adding support for a jsconfig.json in the test runner, as well as adding one to the base directory of the mongo source directory.