`Script_Extensions`: handle unlisted code points

[mathiasbynens]

All code points not explicitly listed for Script_Extensions have as their value the corresponding Script property value.

Ensure this is the case for the generated Unicode data. Currently it looks like we’re ony looking at ScriptExtensions.txt to generate the data.

[mathiasbynens]

https://github.com/mathiasbynens/node-unicode-data/blob/master/scripts/parse-script-extensions.js makes the assumption that for any given script value, its Script_Extensions data is always a superset of its Script data. AFAICT this assumption is correct, but I should probably verify this to make sure.