Add $schema property for devcontainer-feature schema

[Rob9315 (rob9315)]

mirrored how the schema for devcontainer.json does this:

spec/schemas/devContainer.base.schema.json

Lines 10 to 14 in c95ffee

	"$schema": {
	"type": "string",
	"format": "uri",
	"description": "The JSON schema of the `devcontainer.json` file."
	},

to avoid silly warnings like these:

[VolpeDev (TheVolpeDev)]

LGTM;