Support JSON variable files (*.tfvars.json)

[radeksimko]

Use-cases

When declaring variables in a file, users would usually be editing the "native" HCL version and #50 describes how we can help them edit these files.

Variable files generally can also be useful for the language server to provide a pre-computed value of an expression on hover, as described in #495.

Users would benefit from #495 if all variable values were reflected there, i.e. not just *.tfvars, but also *.tfvars.json which would often be generated.

Users would also benefit from an early detection of duplicate variable names across all auto-loaded variable files, i.e.

• terraform.tfvars
• *.auto.tfvars
• terraform.tfvars.json
• *.auto.tfvars.json

Proposal

• Build on the recent PR which added support for *.tf.json Parse *.tf.json for references and symbols #672
• Collect variables from terraform.tfvars.json and *.auto.tfvars.json files in addition to *.tfvars

[github-actions]

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