f/v2

[skibz]

revamping things for a new major revision:

• convert everything to esm (node v14 onwards)
• a more flexible constructor argument pattern supporting snake case, camel case, and radius abbreviations
• support instances of tephra configured to run any permutation of sockets (authentication, accounting, and change of authorisation) instead of all three unconditionally
• improve test coverage

stretch goals:

• revise or get rid of the node_radius_shim stuff. this is by far the clunkiest part of the library... node-radius has unusual marshalling for radius packet attributes, and so we're stuck with dealing with that downstream. we've got to find out whether attributes and vendor attributes can be passed using object notation exclusively.
• fork node-radius... it's practically unmaintained. there are ten year old pull requests outstanding on its bug tracker, and maintenance patches i've submitted were briefly addressed and then ignored for several years. if anything, just for the sake of removing deprecation warnings, and then perhaps we could look at modernising its structure.