RFC: Ion Schema 2.0

[popematt]

Description of Changes
This PR is for a RFC introducing a new version of Ion Schema.

Ion Schema 2.0 will be fully interoperable with Ion Schema 1.0, and includes the following changes:

• Ion Schema Language Versions
• Ion Schema 2.0 Open Content
• ion-schema#18 – Add ieee754_float constraint
• ion-schema#27 – Replace scale constraint with exponent
• ion-schema#38 – Replace nullable:: modifier with $null_or::
• ion-schema#43 – Add a way to require that containers have no duplicate elements
• ion-schema#44 – Allow field names to be constrained without exact field names being specified
• ion-schema#47 – Replace content: closed with closed:: modifier for fields constraint
• ion-schema#51 – Improve modeling of annotations in Ion Schema
• ion-schema#54 – Add support for more ECMA 262 regex features
• ion-schema#58 – Change the default type from any to $any
• ion-schema#72 – Allow timestamp ranges to have endpoints with unknown offsets

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[popematt]

I had an offline conversation about timestamp ranges (see #72), and if we want to support freely mixing known and unknown offset timestamp ranges, it may be a backwards incompatible change, so we may want to include it in this major version update.