Support es6 syntax features part 1

[BasBastian]

The next part of changes

Please check https://github.com/sandflow/imscJS/pull/228 first and then advise. Contains the same set of (possibly) breaking changes

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• based on: feature/transpile-babel-to-umd
• modules remodelled in order to improve readability
• ES6 classes used for TTML doc elements (node-types directory)
• removed "vars", replaced with "const" and "let"
• moved "./names.js" to "./doc-parser/namespaces.js"
• fixed JSDoc comments (they were not JSDoc, because of spaces and asterisks missing)
• reordered variable declarations wherever linter pointed out the issue

[BasBastian]

Might solve https://github.com/sandflow/imscJS/issues/214 and https://github.com/sandflow/imscJS/issues/215

Please check this @palemieux @nigelmegitt

[palemieux]

Replaced by #255