decoder: Support template expressions

[dbanck]

• Part of Recognize individual expressions when nested terraform-ls#496
• Related to Support ${...} template expressions in attribute values terraform-ls#522

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This PR adds support for template expressions inside single and multiline strings.

I filed #345 to track edge cases for empty template expressions inside maps and objects.

Example UX

Directives

if directives within template expressions are turned into conditional expressions and for directives are turned into for expressions. So I would defer support for this until those expressions land

• Support for-in-if expressions terraform-ls#527
• Support conditional expressions (condition ? true : false) terraform-ls#528

[radeksimko]

LGTM aside from the support of HEREDOC in hover and semantic tokens.

I'm happy to finish it given your upcoming OOO though.