WebVMT: Draft Syntax Reference

[rjksmith]

Write a draft syntax reference for WebVMT.

Part of the incubation phase involves recruiting early adopters, which requires the definition of a reference syntax to enable the format to be implemented in a consistent way that permits interoperability.

The Editor's Draft is nearing the point where a minimum viable feature set has been identified to allow independent implementations to be written and encourage the ecosystem of participants to grow, to help progress towards the evaluation phase.

[rjksmith]

Included new Data Model and Syntax sections in the Editor's Draft, with sufficient detail for early adopters to implement functions to import and export VMT files.