Parse *.tf.json for references and symbols

[radeksimko]

Closes #498

Depends on hashicorp/hcl-lang#88

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UX Impact

Completion

References to targetable blocks or attributes (such as variables) declared in JSON are provided to the user as completion candidates in HCL configs.

Hover

Hovering over a reference (such as var.delta) in HCL config provides user details about the reference, such as the type (e.g. string) when the target attribute or block itself is declared in JSON.

Semantic Highlighting

Valid references to targetable blocks or attributes declared in JSON are highlighted accordingly in HCL configs. Note that it may be hard to spot the difference below, but "var." on the screenshot has black+dark-blue colours (highlighted only via static TextMate grammar) or whole dark-blue (semantically highlighted).

Go To References

Using the context menu option "Go to References", the user is able to go from an attribute or a block to any of its references declared in any JSON files.

Go To Definition

Using the context menu option "Go to Definition" or Ctrl/Cmd+click on a reference, the user is able to go from an HCL reference to the definition of a block or an attribute in JSON.

List References

Code lenses providing a number of references to top level blocks continue to be activated on HCL configuration, but will include results from JSON files and (naturally) provide "go-to-reference" pointing to these JSON files.

Symbols

Users is able to find symbols declared in JSON files when seeking for symbols across the workspace.

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