[DESIGN] Bidi usability (#754)

* [DESIGN] Bidi usability

Addresses #746.

DO NOT REVIEW YET

* Update bidi-usability.md

* Add more examples and some use cases

* Update bidi-usability.md

* Add ABNF changes and alternative designs

* Update exploration/bidi-usability.md

Co-authored-by: Mark Davis <[email protected]>

* Update exploration/bidi-usability.md

Co-authored-by: Mark Davis <[email protected]>

* Update exploration/bidi-usability.md

Co-authored-by: Tim Chevalier <[email protected]>

* Add additional user stories, clean up MF2 mentions, add ALM

* Update exploration/bidi-usability.md

Co-authored-by: Tim Chevalier <[email protected]>

* Update exploration/bidi-usability.md

Co-authored-by: Tim Chevalier <[email protected]>

* Update exploration/bidi-usability.md

Co-authored-by: Tim Chevalier <[email protected]>

* Address comments

- replace the ambiguous term `value` with unambiguous terms
  (note that the term value remains for cases where we mean value)
- add @eemeli's alternative considered

* Update bidi-usability.md

- add definitions for LRM/RLM/ALM
- clarify all instances of value
- remove the word 'normalize'
- add example of namespace spillover

* Update bidi-usability.md

* change the LRM/RLM/ALM approach

* improve namespace example

* Update bidi-usability.md

* Add support for isolates in expressions and markup

* Setting line-by-line base direction

* Address comments

Make patterns strictly LTR.
- Only allow LRI/PDI in _expression_ and _markup_
- Require LTR display/edit
- Add an alternative matching my original proposal
- Add illustrations of some of the problems with RTL editing

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Co-authored-by: Mark Davis <[email protected]>
Co-authored-by: Tim Chevalier <[email protected]>

Author: 3 people