Support UTF-8 characters as word delimiters

For a higher usefulness of the utf8strchr function, the index of the
UTF-8 character could be returned in addition with a Rune instead of a
char*.  Since utf8strchr is currently only used by ISDELIM I didn't
bother to increase the complexity.
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Internal Server Error - Gitea: Git with a cup of tea and freedom
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Gitea Version: 1.28.0+dev-505-g31b4d79a84