Add Church Slavonic ('cu') patterns #29

Church Slavonic has the assigned ISO 639 code 'cu'.
Hyphenation patterns for Church Slavonic exist and are open source. They can be downloaded from here. The documentation is here.
Church Slavonic hyphenation is available in TeX (LuaTeX and XeTeX) via the hyph-utf8 package.

One complication is that typically _ (underscore) rather than - (hyphen) is used as the hyphenation character in Church Slavonic texts. I don't see from the Typst documentation if one is allowed to set the hyphenation character. However, the more important issue is loading the hyphenation patterns.