Add "removable" category of characters not required for language support

[simoncozens]

This adds a new category to the exemplar characters called removable. A removable character is a character within base which should be skipped when testing for language support. For example, although fonts should contain a ẞ character to correctly support German, the lack of a ẞ character in a font should not preclude the font from being reported as supporting German.

When choosing whether or not to display a font for a particular language to the end user, the characters in removable should be removed from the base set. When reporting language support to the designer, the characters in base should be used as-is.