Support Unicode normalization

[Witiko]

As discussed with @danopolan in https://github.com/istqborg/istqb_ctfl/commit/5b6e31183b9ead4d58ea2583a2026d1c1a5374e1#commitcomment-141033873, we may want to support the Unicode normalization of markdown input. Since we already use the lua-uni-algos Lua library in the Markdown package, which supports all four Unicode normalization algorithms (NFC, NFD, NFKC, NFKD), we should likely use it. The support would be implemented by a Lua option unicode_normalization of a new option type maybe_string, which would accept values nfc (default), nfd, nkfc, nfkd, and false (or the aliases nil, none, and no in ConTeXt).