Update grapheme cluster width calculation

[tompng]

Implemented rules

Width of NonspacingMark and EnclosingMark is 0
Width of char just after ZeroWidthJoiner is 0
Width of Hangul GraphemeClusterBreak=V,T are 0 because there should be preceding L or LV
Other chars: sum of east asian width for each characters

Relation with grapheme cluster break rules

GB1 - GB5

Break at the start and end of text, unless the text is empty.
Do not break between a CR and LF. Otherwise, break before and after controls.

Not related to width calculation

GB6, GB7, GB8

Do not break Hangul syllable or other conjoining sequences.
L × (L | V | LV | LVT)
GB7 (LV | V) × (V | T)
GB8 (LVT | T) × T

LV LVT are NFC normalized style. L+V, L+V+T are NFD normalized style.
East asian width of L LV LVT is 2. L+V L+V+T should be 2-width, so treat V and T as zero-width.
Width of standalone V and standalone T varies over terminal emulators (0 or 1)
Width of L+V+V+T+T, L+V+T that has no glyph also varies over terminal emulators.

GB9, GB9a, GB9b, GB9c

GB9 × (Extend | ZWJ)
GB9a × SpacingMark
GB9b Prepend ×
GB9c \p{InCB=Consonant} [ \p{InCB=Extend} \p{InCB=Linker} ]* \p{InCB=Linker} [ \p{InCB=Extend} \p{InCB=Linker} ]* × \p{InCB=Consonant}

Extend: Nonspacing Mark | Enclosing Mark | Spacing Mark | halfwidth-dakuten | halfwidth-handakuten | Modifier_Symbol | Format
Linker: Nonspacing Mark

Nonspacing Mark and Enclosing Mark: Zero-width
Character after ZWJ: Zero-width
Other: Follow east asian width
Do nothing for Prepend because width varies over terminal emulators.

GB11

Do not break within emoji modifier sequences or emoji zwj sequences.
GB11 \p{Extended_Pictographic} Extend* ZWJ × \p{Extended_Pictographic}

Assume combined emoji exists, Assume Extend to be Nonspacing Mark.
Character after ZWJ: Zero-width

GB12, GB13

Do not break within emoji flag sequences. That is, do not break between regional indicator (RI) symbols if there is an odd number of RI characters before the break point.
GB12 sot (RI RI)* RI × RI
GB13 [^RI] (RI RI)* RI × RI

Flag emoji width varies over terminal emulators and its configurations. Unable to handle it now.
Width calculation change from 1 to 2 in this pull request.

[ima1zumi]

LGTM, thanks!