Add contextual options to expression-attributes design doc

[eemeli]

The core idea is to provide explicit and well-defined support for u:id, u:locale, and u:dir when used as function or markup options, and to thereby clarify that expression attributes have no runtime impact.

[mihnita]

TLDR: I am happy with namespaced attributes to represent global/universal options (non-function dependent) But that makes the attributes even less useful. So I would suggest we drop them.

[aphillips]

In the 2024-05-20 call, we agreed not to merge this PR but to have @eemeli fold this into exploration/expression-attributes.md design.

[eemeli]

Refactored the PR to be a change/expansion of the expression-attributes design doc rather than a separate one.