Add old Rainbow Brackets extension to dev container

Because VS Code's bracket pair colorization feature doesn't apply
inside docstrings, and hiss has some doctests whose clarity is
improved by colorizing nested matching pairs of parentheses, square
brackets, and curly braces.

References:

- https://stackoverflow.com/q/71093558
- microsoft/vscode#146453
- EliahKagan/palgoviz#191

.devcontainer/devcontainer.json

@@ -8,6 +8,7 @@
     "customizations": {
         "vscode": {
             "extensions": [
+                "2gua.rainbow-brackets",
                 "alefragnani.numbered-bookmarks",
                 "bierner.markdown-preview-github-styles",
                 "eamodio.gitlens",