Report semantic tokens for HEREDOC (multi-line strings)

[radeksimko]

Context

As noted in hashicorp/hcl-lang#186 (comment) we currently report multi-line strings the same way as single-line ones.

Proposal

Consider if reporting multi-line strings (HEREDOC) should be done through a different token, i.e. whether folks may want to highlight HEREDOC differently than simple strings.

Also, in spite of tests suggesting the HEREDOC range is reported correctly, the VS Code token inspector suggests we omit the initial << from the range. This may need to be investigated.