Add the Formattable section to the spec

[eemeli]

As the proposal's text describes, a Formattable is a wrapper around values and their formatting options. It may be thought of as an abstraction that allows us to define the rest of the specification in a manner that's independent of the actual type of any runtime value.

The text briefly refers to MessageFormatParts. The section defining them has not yet been submitted for review, but some relevant discussion may be found in issue #41. Similarly, while PatternElements are also referred to in the preceding PR #195, they are not yet in the spec. Relevant discussion may be found in #190, which also includes some consideration of Formattables.

In the third proposal by @stasm, a corresponding abstraction uses the name RuntimeValue.

[mihnita]

I think we are talking about this in https://github.com/unicode-org/message-format-wg/pull/190 ? So maybe let's not "fork" the discussion and confuse things?

[eemeli]

@mihnita While the discussion in #190 does also cover Formattables, that should not preclude a review of this particular PR.

[eemeli]

Closing as out of date after #307.