feat(core): no runtime envars

[rmlearney-digicatapult]

Core runtime currently depends on DID_VERIFICATION_METHOD envar during resolution through config.ts

Modified to further separate CLI from runtime and turn didwebvh-ts into a pure library

• Modified interface to enable wrapper/caller to provide information about locally controlled DID(s)
• CLI updated to use new interface
• Move config.ts methods into CLI (not required for runtime)
• CLI tests now use temporary directory for further separation
• Deleted dead envars
• Updated documentation around wrapper/caller use of interface

[rmlearney-digicatapult]

Needs #138 first