Add split-grapheme-cluster and string normalization function

[puripuri2100]

• Add split-grapheme-cluster : string -> string list
• String normalization when parsing strings and inline-text

[gfngfn]

Thanks for making a PR! It looks basically good!

I think the following API for strings may be a better formalization:

• val normalize-string-to-nfc : string -> string
• val normalize-string-to-nfd : string -> string

in that this:

• allows users to use other normalization forms than NFC, such as NFD, and
• does not require that string values are always internally NFC, and thereby keeps the implementation less error-prone.

Would you agree with this kind of API design?

[puripuri2100]

I agree it. I fixed the code.

[gfngfn]

I've re-read this PR and found it to be basically good. Thank you!