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Fixes #2941
This fixes the rendering of the text below. For those that can't see it,
it is the following in UTF-32: `0x22 0x1F3FF 0x22`.
```
"🏿"
```
`0x1F3FF` is the Fitzpatrick modifier for dark skin tone. It has the
Unicode property `Emoji_Modifier`. Emoji modifiers are defined in UTS
#51 and are only valid based on ED-13:
```
emoji_modifier_sequence := emoji_modifier_base emoji_modifier
emoji_modifier_base := \p{Emoji_Modifier_Base}
emoji_modifier := \p{Emoji_Modifier}
```
Additional quote from UTS #51:
> To have an effect on an emoji, an emoji modifier must immediately follow
> that base emoji character. Emoji presentation selectors are neither needed
> nor recommended for emoji characters when they are followed by emoji
> modifiers, and should not be used in newly generated emoji modifier
> sequences; the emoji modifier automatically implies the emoji presentation
> style.
Our precomputed grapheme break table was mistakingly not following this
rule. This commit fixes that by adding a check for that every
`Emoji_Modifier` character must be preceded by an `Emoji_Modifier_Base`.
This only has a cost during compilation (table generation). The runtime
cost is identical; the table size didn't increase since we had leftover
bits we could use.
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