Add writing support for lookup table 7 (Extension Substitution Subtables)

[Connum]

PR #486 will add parsing support for the extension tables, but we'll still need writing support.

In detail we'll have to make sure:

1. That tables marked as format 7 will be written instead of throwing an error that they are unsupported
2. when writing tables (e.g. added manually/programmatically), we'll have to check whether any table offset would go beyond the 16-bit range and if so, add a lookup table 7 for that table