Semantic skeletons design (#1067)

* Semantic skeletons design

This design document contains the proposed design for including semantic skeletons into Unicode MessageFormat's functions.

This PR is based on @sffc's [doc](https://docs.google.com/document/d/1s7GeN5V0cnw9B1erfMHTWwmnxz0EJkq2v78ZjzRq2t8/edit)

* Add links

* Explain picture strings better

* Update exploration/semantic-skeletons.md

Co-authored-by: Eemeli Aro <[email protected]>

* Replace picture output with a table

* Fix objective section

* Update semantic-skeletons.md

* Fix requirements per comments

- Put all the time types together in a single requirement about operands

* Update semantic-skeletons.md

* Update exploration/semantic-skeletons.md

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

* Update exploration/semantic-skeletons.md

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

* Update exploration/semantic-skeletons.md

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

* clarify the picture string table

Address @duerst's comment

* Update exploration/semantic-skeletons.md

Co-authored-by: Eemeli Aro <[email protected]>

* Add FAQ, improve option bag skeleton description

* An apostrophe escaped notice

* Update exploration/semantic-skeletons.md

Co-authored-by: Eemeli Aro <[email protected]>

* Adding use cases and descriptions of types

* Add an alternative design

* Update exploration/semantic-skeletons.md

Co-authored-by: Eemeli Aro <[email protected]>

* Update exploration/semantic-skeletons.md

Co-authored-by: Eemeli Aro <[email protected]>

* Update exploration/semantic-skeletons.md

Co-authored-by: Eemeli Aro <[email protected]>

* Make `Intl.DateTimeFormat` present tense

* typo

* Add @eemeli's example

* Apply suggestion manually (time zone/offset discussion)

* Update exploration/semantic-skeletons.md

* Start work on 2025-05-12 discussion

Start working on error section, add references to ICU4X field sets, including a table

* Add materials about field width specification

* Add design from @sffc's comment

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

Author: 3 people