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.

src/snapshots/osdb__command__tests__execute_insert_statement.snap

@@ -2,4 +2,4 @@
 source: src/command.rs
 expression: result
 ---
-insert
+insert 45 "user" "user@example.org"