Create access helper to support path references up stack

Add a new {@access key="path" /} helper that allows referencing paths that are up stack from the current context. Add a bunch of tests to cover use cases.  Sample usages are:

{@access key="a.b.c" /}
{@access key="a.b[2]" /}

Though not the intended use case paths like ".", ".name", "name" are allowed and return the same as {.}, {.name}, {name} although the latter are the preferred ways to do these things.

JSdoc is in the code. Wiki doc will be added after PR is accepted.