Formally define (ABNF) 3.x URL templating behavior

[handrews]

As noted in Slack discussions, there is no formal specification for where variables can appear in server URL templates and path templates. There does not seem to be any reason to place restrictions on it.

Ideally, we could leverage the ABNF of RFC 6570 URI Templates even though OAS 3.x templating is not the same as RFC 6570 in general. (@darrelmiller @webron @MikeRalphson any concerns here?)

[handrews]

Issue #2119 brings up a lot of points relevant to this.