Feature: Add support for customizations

Our projects contain `devcontainer.json` specification with customizations for useful extensions and dev-experience type settings like:
```
"customizations": {
"vscode": {
"extensions": [
"dbaeumer.vscode-eslint",
"firsttris.vscode-jest-runner",
"gamunu.vscode-yarn",
"github.vscode-pull-request-github",
"github.vscode-github-actions",
"infeng.vscode-react-typescript",
"pmneo.tsimporter",
"redhat.vscode-yaml",
"yoavbls.pretty-ts-errors",
"yzhang.markdown-all-in-one"
],
"settings": {
"editor.formatOnSave": true,
"terminal.integrated.defaultProfile.linux": "zsh"
}
}
```

When using envbuilder with Coder, these are not loaded into the Coder development environments.

It would be really useful to support the `customizations.vscode` devcontainer settings.

Start by tracing how envbuilder reads devcontainer.json and where development-environment settings are applied. Compare the existing handling with the customizations.vscode structure shown in the issue; done means those VS Code extensions and settings are loaded into Coder development environments.

Written by the indexing model from the issue text.