Progress: Add is joiner UTF-8 function.

Progress: Add is joiner UTF-8 function.

I figured this would eventually happen given my relative inexperience with UTF-8.
This adds a new feature via additional functions for explicitly checking for joiner characters.
These joiner characters are not the same as combining characters.

The is alphabetic tests need to also handle joiner characters and treat them as not alphabetic.

This requires various build settings, like stand alone builds, to be updated with one of the following source files added:
```
  fll/level_0/private-utf_joiner.c
  level_0/private-utf_joiner.c
  private-utf_joiner.c
```

This requires various `config.h` files to now support handling of:
```
  #define _di_f_utf_character_is_joiner_
  #define _di_f_utf_is_joiner_
```

I had the option of either adding this and not exposing it through the **API** or exposing it through the **API**.
Exposing it through the **API** causes this to be a new feature (which I really want to avoid in general).
However, if the code is going to be added (because it is necessary and fixes some bugs), then it makes more sense to just expose it through the **API**.

This also fixes incidental missing `_di*_` in some cases.