Use standalone lark parser

[Yuri Astrakhan]

This amazing library would often run in a highly secure environment, e.g. various infrastructure automation env. As such, it would be great if python-hcl2 used no external dependencies to simplify security audit.

From documentation, Lark can generate parser code directly, which would result in a hcl2-specific python file that can be added directly to the library, while having exactly the same interface as the full Lark lib. I also suspect that the parser will be faster than when parsing using declarative rules. Thanks!