fix(devcontainer): add editorconfig vscode extension

[in-a-dil-emma]

VS Code doesn't honour .editorconfig without this extension.

Summary

• What changed: Added EditorConfig.EditorConfig to vscode extensions list in devcontainer.json
• Why it is correct: VS Code does not honour .editorconfig without the appropriate extension.
• Risk: None.

Verification

• Tests run: None.
• Manual checks: Opening files at random and checking that .editorconfig is honoured.
• Screenshots or recordings: See below.

Before

After

As seen here, the tab width setting for Dockerfile in .editorconfig is now honoured.

Checklist

• I understand every change in this PR.
• I can explain what it does and why it is correct.
• I disclosed any LLM coding help below.

LLM Disclosure

I used Human Intelligence.