Add old Rainbow Brackets extension to dev container

Because VS Code's bracket pair colorization feature doesn't apply
inside docstrings, and palgoviz has a lot of 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

.devcontainer/devcontainer.json

@@ -12,6 +12,7 @@
     "customizations": {
         "vscode": {
             "extensions": [
+                "2gua.rainbow-brackets",
                 "alefragnani.numbered-bookmarks",
                 "bierner.markdown-preview-github-styles",
                 "eamodio.gitlens",