feat(doris): make lexer case insensitive (#44)

Add `options { caseInsensitive = true; }` to DorisLexer.g4 to support
lowercase SQL keywords like `show`, `select`, `from`, etc.

Also add regression tests:
- routine_load.sql: lowercase `show create routine load`
- subquery_comparison.sql: subquery in comparison expression

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: h3n4l

doris/DorisLexer.g4

@@ -19,6 +19,8 @@
 
 lexer grammar DorisLexer;
 
+options { caseInsensitive = true; }
+
 // @members section removed - Java code not compatible with Go target
 // Original predicates isValidDecimal() and isNoBackslashEscapes removed