Dev Containers: support VS Code customizations

[jmcphers]

System details:

Positron and OS details:

Positron Version: 2026.01.0 build 79
Code - OSS Version: 1.106.0
Commit: 79aa84557e00e07bff8c7a6ba5adaa7bc1a6ef5b
Date: 2025-12-11T07:22:57.765Z
Electron: 37.7.0
Chromium: 138.0.7204.251
Node.js: 22.20.0
V8: 13.8.258.32-electron.0
OS: Darwin arm64 25.1.0

Session details:

N/A

Describe the issue:

VS Code Customizations (extensions and settings) aren't supported in Positron's Dev Containers implementation.

These customizations aren't formally part of the Dev Container spec; it simply provides for tools to define their own customizations:

https://containers.dev/implementors/json_reference/#image-specific

The actual VS Code customizations are documented here:

https://containers.dev/supporting#visual-studio-code

Steps to reproduce the issue:

Create a dev container file with a VS code customization, e.g. from https://code.visualstudio.com/docs/devcontainers/containers#_create-a-devcontainerjson-file

{
  "image": "mcr.microsoft.com/devcontainers/typescript-node",
  "forwardPorts": [3000],
  "customizations": {
    // Configure properties specific to VS Code.
    "vscode": {
      // Add the IDs of extensions you want installed when the container is created.
      "extensions": ["streetsidesoftware.code-spell-checker"]
    }
  }
}

Open the devcontainer in Positron and observe that the customizations are not applied.

Expected or desired behavior:

VS Code customizations also apply to Positron.

It's debatable whether we should use a positron key instead of (or in addition to) vscode, but I suspect most people using devcontainers will be sharing with people using VS Code, so reading the vscode key seems like a good start.