feat(parser): implement insert command parsing

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.

notes.org

@@ -235,6 +235,8 @@ i will use rustyline, since it seems like the most feature-complete
 insert <id:int> <username:string> <email:string>
 ** TODO Row struct
 ** TODO parse row insert
+** TODO separate statements with semicolons
+** TODO in case of parse error, skip until next semicolon to better recover
 ** TODO serialize/deserialize row to/from raw bytes
 *** TODO look for best practices for creating binary formats