R1:9bdf477f2fef

Combine the two different ngram-splitting algorithms into a single engine

Summary:
Ref T13501. Depends on D21127. With the "prefix" behavior removed in D21127, we now have two virtually identical copies of the same code.

The newer one in Ferret is better: it slices utf8 correctly and is slightly more efficient on large inputs. Pull it out and make all callers call into it.

Test Plan:
- Grepped for all affected symbols.
- Ran `bin/search index --force ...` to reindex various objects (tasks, files).