Add detailed line-breaking rules.

[r12a]

This is a comment from the i18n WG, however we recognise that it comes late in the process for 2nd Edition, so we recommend that if it is not fixable before your transition, you consider it when you start work on the 3rd Edition.

D.1.51 #lineBreak-uax14
https://www.w3.org/TR/2018/REC-ttml1-20181108/#feature-lineBreak-uax14

There appears to be no way to customise or control line breaking on a language by language basis.

Also, it seems unclear what lineBreak-uax14 is and why it's called lineBreak-uax14 rather than just linebreak with a link to UAX14 as a fallback method of determining line breaks, but allowing for alternative tailorings on a language-by-language basis.

[skynavga]

N.B. According to the new TTML architecture rules, the addition of new substantive functionality would have to be accomplished as a TTML extension (as opposed to a feature) and accomplished in a module specification as opposed to the TTML core specification; however, in the mean time, this is being left as marked for 3ED milestone of TTML2 as a place holder.