OPENNLP-1885: Add WordpieceEncoder and fold the unreleased BertTokenizer into it

WordpieceEncoder in opennlp-api runs the full BERT tokenization pipeline
as a SubwordTokenizer: every piece carries its vocabulary id and the span
of the original text, surviving the normalization steps that change,
insert, and remove characters. Content is computed with the same library
calls the previous pipeline made; offsets come from a per-code-point
rerun, with contextual case mappings (Greek final sigma) falling back to
word-wide spans that widen but never misplace. List and map constructors
cover line-number and explicit-id vocabularies.

BertTokenizer, unreleased and superseded, is removed. The dl tokenizer
creation builds on the encoder behind the existing Tokenizer plumbing via
a package-private adapter with unchanged special-token selection, and a
vocabulary missing its special tokens now fails at construction instead
of at the first id mapping, pinned by a test.

Parity is enforced twice: a differential suite against the reference
pipeline (kept test-only as ReferenceBertPipeline) over a curated corpus
plus 800 randomized inputs, and the removed class's reference token
sequences ported case for case. WordpieceTokenizer is untouched.

Author: krickert

opennlp-api/src/main/java/opennlp/tools/tokenize/BertNormalization.java

@@ -19,7 +19,7 @@
 
 /**
  * Character classifications and text transforms of the reference BERT
- * {@code BasicTokenizer}, shared by {@link BertTokenizer} and
+ * {@code BasicTokenizer}, shared by {@link WordpieceEncoder} and
  * {@link WordpieceTokenizer}.
  */
 final class BertNormalization {