Disallow unrecognized options in @truffle/config

[benjamincburns]

Truffle should error when it receives a config field that is unrecognized. Doing this helps users to spot typographical errors in field names.

Unfortunately this change may make things more challenging for plugin developers who want users to be able to specify additional fields in the config object for their plugin to use. If this is the case, then as part of this change it should be possible for plugins to specify their config requirements so that Truffle can check it accordingly.

Note: This was captured from an internal session where we discussed potential future breaking changes for major releases. Until the team assigns this ticket, this is merely a proposal, and not a commitment to make this specific change.