Enable language backends

https://www.pantsbuild.org/docs/initial-configuration\#enable-backends

This does not add any lint/fmt/etc backends, just the language-specific ones.

Author: cognifloyd

pants.toml

@@ -1,5 +1,14 @@
 [GLOBAL]
 pants_version = "2.13.0rc1"
+backend_packages = [
+  # python
+  "pants.backend.python",
+  "pants.backend.experimental.python",
+  "pants.backend.python.mixed_interpreter_constraints",
+
+  # shell
+  "pants.backend.shell",
+]
 
 [source]
 # recording each pack individually under root patterns is not great, but resolves these issues: