Allow expressions in string interpolation

[zabacad]

HCL allows for any expression in a string interpolation. This updates the grammar and transformer to match, allowing for arbitrarily-deep nesting.

[piotrb]

I just want to throw my 2c here, I used this patch as part of an internal tool that parses terragrunt hcl files to generate stack information for setting up spacelift stacks :) and this patch definitely works :) .. it does of course change the internal structure of the lark nodes being returned, but it works great to solve the issue at hand around supporting expressions inside interpolations.

[Qantas94Heavy]

Is there anything blocking this from being merged? This is impeding our use of python-hcl2 in our internal tooling and patching/forking this would be a lot of overhead for us.