hclsyntax: Recovery of incomplete parenthesis

## Context

Similar to some earlier issues, we ran into an edge case while implementing support for operators and parenthesis (`ParenthesesExpr`) in the Terraform language server.

```hcl
attr = (
```

This is parsed as following:

```go
hclsyntax.Attribute{
Name: "attr",
Expr: &hclsyntax.LiteralValueExpr{
SrcRange: hcl.Range{
Start: hcl.Pos {Line: 2, Column: 1, Byte: 9},
End: hcl.Pos {Line: 2, Column: 1, Byte: 9}
}
},
SrcRange: hcl.Range{
Start: hcl.Pos {Line: 1, Column: 1, Byte: 0},
End: hcl.Pos {Line: 2, Column: 1, Byte: 9}
}
}
```

So while we can tell we are inside of an attribute and that the expression is part of the attribute - i.e. within its range, it remains awkward/difficult to figure out whether we are inside of the expression.

As with other reports related to recovery from incomplete configuration, the context is - again - enabling completion. Users would reasonably expect to be offered relevant completion in the above context, because it is exactly the nature of completion to complete the incomplete configuration.

## Proposal

Report/parse the above incomplete expression with the following range:

```go
hcl.Range{
Start: hcl.Pos {Line: 1, Column: 8, Byte: 7},
End: hcl.Pos {Line: 2, Column: 1, Byte: 9}
}
```

Start by tracing hclsyntax parsing for incomplete ParenthesesExpr expressions and compare it with the existing recovery cases referenced in the issue. Reproduce the `attr = (` example, then verify that the incomplete expression receives the proposed range from line 1, column 8, byte 7 through line 2, column 1, byte 9.

Written by the indexing model from the issue text.