hclwrite: Duplicate functions NewFile and NewEmptyFile

`hclwrite.NewFile` and `hclwrite.NewEmptyFile` are, on closer inspection, functionally identical duplicate code.

Suggestion: Replace `NewFile` (which manually reimplements `newBody` instead of invoking it) with a trivial deprecated wrapper invoking `NewEmptyFile`. (Or, perhaps, keep the `NewFile` name, along with the shorter more concise implementation from `NewEmptyFile`?)

Another suggestion: Rename hclwrite's public.go to helpers.go, since it isn't the only file defining public symbols.

Start by comparing hclwrite.NewFile and hclwrite.NewEmptyFile, then inspect hclwrite/public.go and the surrounding public symbols. Resolve whether NewFile should remain as a deprecated wrapper or retain its name while adopting the shorter implementation; done means the duplicate logic and file-naming concern have a clear, compatible resolution.

Written by the indexing model from the issue text.