Support plus operator for string concatenation

Inspired by code in https://github.com/VladRassokhin/intellij-hcl/issues/212

```hcl2
resource "aws_kms_alias" "main" {
name = "alias/${var.alias}"
}
```

```hcl2
resource "aws_kms_alias" "main" {
name = "alias/" + var.alias
}
```

Should be equivalent, though `terraform validate` shows an error
`Unsuitable value for left operand: a number is required.`

Start by reproducing the two HCL examples and the reported `terraform validate` error, then trace how expression operators are parsed and evaluated in this repository. The work is done when string concatenation with `+` behaves equivalently to interpolation and the relevant validation behavior is covered.

Written by the indexing model from the issue text.