Update extensions list and location

[unional]

Currently, the template has the extensions recommendation (and settings) in the .devcontainer/devcontainer.json.
They only apply within a container, meaning if the user develop directly in their native env, they are not available.

It's better to move them to .vscode/extensions.json and .vscode/settings.json.

Also, the extension recommendations list both github.vscode-github-actions and me-dutour-mathieu.vscode-github-actions. Maybe that should be deduped?