Identify Cargo.toml and Cargo.lock as cargo and cargo-lock

Introduce dedicated types for Cargo.toml and Cargo.lock files. These are
similar to go.mod and go.sum files.

Author: jalaziz

identify/extensions.py

@@ -341,6 +341,8 @@
     '.zshenv': EXTENSIONS['zsh'],
     'AUTHORS': EXTENSIONS['txt'],
     'BUILD': EXTENSIONS['bzl'],
+    'Cargo.toml': EXTENSIONS['toml'] | {'cargo'},
+    'Cargo.lock': EXTENSIONS['toml'] | {'cargo-lock'},
     'CMakeLists.txt': EXTENSIONS['cmake'],
     'CHANGELOG': EXTENSIONS['txt'],
     'config.ru': EXTENSIONS['rb'],