Fix BAST Alternation bug, add comprehensive tests, clean up deprecated files

BAST Fixes:
- Fix Alternation deserialization by adding readTypeExpressionContents() helper
- readNode() handles NODE_* tags, readTypeExpression() handles TYPE_* tags
- These are disjoint sets - mixing them caused byte stream misalignment

Test Suite (54 BAST tests):
- Add BASTIncrementalTest with 37 test cases from simple to complex
- Add BASTMinimalTest, BASTDebugTest, BASTPerformanceBenchmark
- Add BASTWriterSpec, BASTBenchmarkRunner
- Add comprehensive-test.riddl test resource

Cleanup:
- Remove deprecated bast/shared source files (code lives in language/bast/)
- Add npm-packages/.gitignore
- Add examples/npm-usage for npm package usage examples

All 244 passes module tests pass.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: reidspencer