Add support for semicolon-terminated statements according to the
updated grammar. This change enables executing multiple SQL statements
in a single input by separating them with semicolons. Key improvements
include:
- Update grammar to require semicolons after statements
- Add Semicolon token to the tokenizer
- Implement error recovery by skipping to next semicolon on parse errors
- Create helper functions for checking semicolons in statement parsers
- Add tests for multiple statements and error conditions
Implements the parse_insert_command function to handle the insert
statement according to the grammar definition. The function now
correctly parses the command format insert int string string and creates
a structured Statement::Insert with the id, username, and email fields.
