Support project file modifications via LSP

### Description

FSAC provides LSP commands for project file modifications that prevent client applications from having to learn the details of this format. It would be nice to have integration for:

* fsproj/moveFileUp
* fsproj/moveFileDown
* fsproj/addFileAbove
* fsproj/addFileBelow
* fsproj/addFile
* fsproj/removeFile

The moveFileUp/moveFileDown/addFile/removeFile commands all use the following structure:

```fsharp
type DotnetFileRequest =
{ FsProj: string // absolute path to the project file (not a file:// uri)
FileVirtualPath: string // relative path (from the parent directory of the FsProj above) specifying the file to act on
}
```

The addFileAbove/addFileBelow commands use the following structure:

```fsharp
type DotnetFile2Request =
{ FsProj: string // absolute path to the project file (not a file:// uri)
FileVirtualPath: string // relative path (from the parent directory of the FsProj above) specifying the file the new file will be added above/below
NewFile: string // relative path (from the FileVirtualPath above) specifying the new file to create
}
```