Support users being able to configure Terraform input variables through the editor

[hugorut]

Infracost VSCode has a naive way of offering users the ability to configure Terraform input variables. If terraform var files with the names terraform.tfvars or *.auto.tfvars are found, these are used as input files. We should investigate how we can improve this experience for users. Initial thoughts are:

1. We could allow users to specify names of input var files
2. Offer the ability to pass in variables for the top-level variables, perhaps through a form.

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