docs(common): add extended string binary specification to kmx-file-format.md

Noted that the whole UC_SENTINEL / CODE_xxx / ... extended string format is missing from this file. Possibly helpful for @markcsinclair if we can get this documented!

Extended strings are a C-style zero-terminated UTF-16LE string. They can contain variable-length tokens to embed additional functionality. These are prefixed with a UC_SENTINEL codepoint, then a CODE_xxxx codepoint, and then 0 or more parameters.

See:

https://github.com/keymanapp/keyman/blob/5b1c853557b4412bf3d9c456fac8deb264dfde90/common/include/kmx_file.h#L181-L225