Add token type examples to make error messages more helpful. Created an
ExpectedToken enum to replace string literals for better type safety,
added example values for each token type, and enhanced error display to
show concrete examples of valid syntax.
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.
