feat(IO.Path): add `addPostfix` function

```fsharp
let addPostfix postfix (path: string) =
System.String.Concat [|
Path.GetFileNameWithoutExtension path
postfix
Path.GetExtension path
|]
```

Start by locating the IO.Path module or entry point in the FsharpMyExtension repository and review nearby path helpers and tests, if present. Add the requested addPostfix function with the behavior shown in the issue, then run the project's existing build or test command to confirm it is available and works with file names and extensions.

Written by the indexing model from the issue text.