add a 'json.validate' command

[aeschli]

Allow to validate JSON from core until we have a better solution

When run from core:

		const schemaUri = URI.parse('https://json-schema.org/draft-07/schema');
		const content = JSON.stringify({ type: 'foo' });
		commandService.executeCommand('json.validate', schemaUri, content).then(result => {
			console.log('Validation result', result);
		});

Result:

[
    {
        "severity": "Warning",
        "message": "Value is not accepted. Valid values: \"array\", \"boolean\", \"integer\", \"null\", \"number\", \"object\", \"string\".",
        "range": [
            {
                "line": 0,
                "character": 8
            },
            {
                "line": 0,
                "character": 13
            }
        ],
        "code": 1
    }
]