decimal-format and format-number are not implemented

[jbowtie]

These are the last bits that need to be implemented for spec compliance.

[kivutar]

Example implementation in libxslt:

https://github.com/GNOME/libxslt/blob/840c8e2b6ab33a02374d7a097868649e492da768/libxslt/functions.c#L601

https://github.com/GNOME/libxslt/blob/32c88216ddbaa0f3491f45bc84ee89285c6a1129/libxslt/numbers.c