Add design doc on function composition (#753)

* Add design doc on function composition

* Add date

* Add PR number

* Clarify that :number is meant to be an illustrative example

* Define 'Formattable' and 'FormattedValue' terms and use terms in naming areas of ambiguity

* Add a requirement to identify supported and unsupported examples; taxonomize possible solutions

* Add requirement about staying in touch with user needs

* Add links to past work

* Add 'prior work' section

* Add a fourth alternative under proposed designs

* Update exploration/function-composition-part-1.md

Co-authored-by: Elango Cheran <elango@unicode.org>

* Update exploration/function-composition-part-1.md

Co-authored-by: Elango Cheran <elango@unicode.org>

* Minor phrasing change

* Clarify steps under 'Ambiguous examples'

* Address feedback from Addison

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Co-authored-by: Elango Cheran <elango@unicode.org>