replace `InCB_Extend_table` search with simple predicates (#178)

According to [UCD](https://www.unicode.org/reports/tr44/),
the property `InCB=Extend` is defined as:
```
\p{InCB = Extend} ≔ [
\p{gcb=Extend}
\p{gcb=ZWJ}
- \p{InCB=Linker}
- \p{InCB=Consonant}
- [\u200C]
]
```

And no `InCB=Consonant` is `gcb=Extend` or `gcb=ZWJ`.
So in can be determined with the grapheme category the caller already has and two equality.

`scripts/unicode.py` now checks that derivation against the UCD.
A future Unicode version that breaks either fact fails the generator
instead of silently mis-segmenting Indic text.

Also one fact that is `InCB=Linker` and `InCB=Extend` are subset of
`gcb=Extend`, any other category rules out both without inspecting the codepoint.

This change improves the overall performance of the grapheme APIs
by skipping unnecessary category checks and table searches in the hot loop.
On my machine, it measures approximately -15 to -20%.