Switch segmentation plugin ABI to codepoint lengths

[frankslin]

Update the segmentation plugin ABI entry point to:

• opencc_get_segmentation_plugin_v2()

Segmentation results are returned as a sequence of segment lengths measured in
Unicode code points, not as copied token strings. The ABI contract is:

• input text is passed to the plugin as null-terminated UTF-8
• the plugin returns segment_count plus codepoint_lengths
• each element is the number of Unicode code points in the next segment
• lengths must be positive and must cover the full input, in order
• the host reconstructs segment boundaries from the original UTF-8 input

This keeps the ABI simpler and avoids allocating one string per token across
the plugin boundary.

[BYVoid]

Add description?

[frankslin]

Add description?

Done.