Feature: Add support for customizations

[lornest]

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.