add a small utility program to test the valid API keys in .env file

[bmerkle]

Hi there,

the setup of TypeAgent involves quite some API keys for different services.
The services are described in README.md#service-keys

I will create a small test program which

• verifies the service keys or their usage (so that is working)
• test each service separately
• test only services for which the key=value is present in the .env file
• if key=value are not in the .env file the program will put out an informative warning and show a small example of the expected value
• if the test fails, the program will give a descriptive error message and also include the expected format of the value for the key

I will create a PR which implements this behaviour