TTML2 Repo - New Labels - 'syntax', 'semantics'

I have added two new labels to substantive, merged PRs in the 2ED-FPWD
milestone [1]:

   - syntax
   - semantics

which characterize the nature of the change and its potential impact on
authors and implementations.

Under the heading of *syntax* changes are included the following:

   - adding (increasing) constraints, e.g.,
      - eliminating the use of an element in some context
      - eliminating the use of a character in some #PCDATA content context
      - eliminating the use of an attribute or attribute value in some
      context
   - removing (reducing) constraints. e.g.,
      - allowing the use of an element in some context
      - allowing the use of a character in some #PCDATA content context
      - allowing the use of an attribute or attribute value in some context

Changes to syntax affect document conformance. Consequently, these changes
can affect existing content and implementations (particularly, validating
processors).

In contrast, substantive changes that do not affect *syntax* are all
related to changes in *semantics*, for example,

   - changing the default value of an attribute
   - changing the definition of a feature designation
   - changing the definition of a processing function, procedure, or
   algorithm

At present, 4 merged substantive PRs are labeled as *syntax*, while the
remaining 8 are labeled as *semantics*. I next plan to add labels related
to testing status.

[1]
https://github.com/w3c/ttml2/pulls?utf8=%E2%9C%93&q=is%3Apr+is%3Aclosed+is%3Amerged+milestone%3A2ED-FPWD+label%3Asubstantive+