Fixed issue described in inexorabletash/text-encoding/issues/67
The problem was casued by the missing propery accessor on the
require statement, which passed the entire global scope of
`encoding-indexes` instead of the indexes.

Solve a transpiling issue with Babel where it may be evaluated in contexts with no global `this` defined.

Don't expect encoding-indexes to return object with indexes & don't require them if they are already present (#61) - Resolves #60

* Don't require encoding-indexes if they are already present
* Don't expect encoding-indexes to return object with indexes

* Shift_JIS encoder: exclude certain jis0208 index entries
* Big5 encoder: exclude HKSCS extensions
* Align comments with spec text around "inclusive"
* Align comments w/ spec capitalization of Big5 Shift_JIS
* Match spec math tweaks (e.g. const - const + pointer)

Performance improvements

Add valid SPDX license identifier