Add option to opt out of extensions

[chrmarti]

Tracking devcontainers/features#386 here.

[chrmarti]

I suggest we use the - prefix (suggested in #415) to omit extensions from being installed. This fits in the existing devcontainer.json and devcontainer-feature.json customizations, e.g., omitting one while adding another extension would look like:

"customizations": {
	"vscode": {
		"extensions": {
			"-dbaeumer.vscode-eslint",
			"ms-python.python"
		}
	}
}

Additions / removals of extensions in the devcontainer.json should take precedence over those in the devcontainer-feature.jsons and these should both take precedence over those in the base image.

[chrmarti]

Available in Dev Containers 0.323.0-pre-release.