Unit Testing and Exception Handling ++ (Fixes #50)

[pmartindev]

• Added unit tests for the synthesizing of the mutato.yml file, string, and object
• Added unit tests for verifying that the appropriate resources are synthesized from the Mutato object
• Added errors.ts to add exception handling in app.ts
• Modified the devcontainer to import the env. vars. in mutato.env
• Abstracted the lengthy functions to synthesizeHelpers.ts as an object for better readability/maintainability
• Fixed misc. spelling errors