Add shellcheck extension to dev container (radius-project#7963)

# Description

Adds the shell check extension to the dev container. See
https://marketplace.visualstudio.com/items?itemName=timonwong.shellcheck.

## Type of change

- This pull request is a minor refactor, code cleanup, test improvement,
or other maintenance task and doesn't change the functionality of Radius
(issue link optional).

Fixes: N/A

@rynowak FYI

Signed-off-by: Brooke Hamilton <45323234+brooke-hamilton@users.noreply.github.com>

.devcontainer/contributor/devcontainer.json

@@ -37,7 +37,8 @@
         "ms-azuretools.vscode-bicep",
         "ms-kubernetes-tools.vscode-kubernetes-tools",
         "ms-azuretools.vscode-dapr",
-        "ms-vscode.makefile-tools"
+        "ms-vscode.makefile-tools",
+        "timonwong.shellcheck"
       ]
     }
   },