Batch Syntax Highlighting - support multiline comments with `::` and `^`

[Andrej730]

Crossposting from microsoft/vscode#223115 as this seems to be the source for cmd syntax highlighting in VS Code.

Steps to Reproduce:

1. There is a way to create multiline comments in batch files by combining :: and ^ (to create multiline commands).

:: multiline comment ^
echo hi, it's still multiline comment ^
and still...
@echo it's printed as multiline comment is over.

2. See the syntax highlight, lines echo hi, it's still multiline comment ^ and and still... are not greyed out as comments though they kind of are.