Port code from terragrunt-langserver

[yhakbar]

As discussed with @mightyguava, we'll be porting over content from this project:
https://github.com/mightyguava/terragrunt-langserver

To the terragrunt-ls repo.

Content to port:

1. AST index for accurate targeting of nodes in Terragrunt configurations.
2. VS Code extension
3. CI/CD for VS Code extension.

[yhakbar]

The AST indexing has been pulled in.