Provider of FoldType constants for the MimeType.
The Provider should enumerate FoldTypes that apply to the given MIME type.
There can be multiple providers for a MIME type - some advanced constructions in
the language can be recognized / folded by extension modules. Consider Java vs.
Bean Patterns, or XML vs. Spring bean config.
FoldTypes will be collected and some pieces of UI can present the folds, such
as Auto-folding options.
The Provider may specify inheritable=true; in that case the contributed FoldTypes
will become available for more specific MIME types, too. For example, if a FoldTypeProvider
for text/xml registers FoldTypes TAG and COMMENT with inheritable=true,
those FoldTypes will be listed also for text/x-ant+xml. This feature allows
to "inject" Fold types and FoldManager on general MIME type ("") for all
types of files.