Consider using end pattern as bail out for embedded languages

This started as a discussion in https://github.com/jlelong/vscode-latex-basics/issues/58.

The problem is that languages that are embedded within other languages have a high potential to not work as expected. When there's a problem in the embedded grammar it can easily escape into the primary grammar.

Key comment from @jlelong copied below:

@jlelong would this be something that you would define in your textmate grammar at the point that you embed the grammar?

@alexr00 yes exactly. For instance, embedding the Python language typically looks like

{
	"begin": "some_begin_pattern",
	"end": "some_end_pattern",
	"contentName": "source.python",
	"patterns": [
		{
			"include": "source.python"
		}
	]
}

It would be nice if the top level grammar could escape from the Python one when the end pattern is reached. I originally thought a new pattern was needed to trigger the bail out but I believe it will always be the same as the end pattern.

Currently, we provide a modified C++ grammar which includes the bail out pattern. This approach would make it useless. Embedding a grammar would much more robust.