Support grapheme clusters when calculating width

[gaborcsardi]

Happening in #316. Functions to update:

• ansi_align() will be good, once ansi_nchar() is good.
• ansi_chartr() should this work on code points or graphemes? Both? #323
• ansi_columns() probably only needs ansi_nchar(), but need to test.
• ansi_has_any() is good.
• ansi_html() is good.
• ansi_nchar() to default to graphemes when using type = "chars". (type = "width" is fine, because it calls utf8_display_width().)
• ansi_simplify() is good.
• ansi_strip() is good.
• ansi_strsplit() with grapheme support, when the separator is the empty string. #326
• ansi_strtrim() could use a better implementation. #324
• ansi_strwrap() needs a better implementation, that does not rely on base::strwrap() #325
• ansi_substr()
• ansi_substring() will be OK once ansi_substr() is done.
• ansi_tolower() probably needs a better Unicode implementation. Might not happen in this PR.
• ansi_toupper() the same.