Support project file modifications via LSP

[baronfel]

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:

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:

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
  }

[amcguier]

I took a stab at this that works pretty well for me (I've been running it in a private fork for a while). There are some design choices that I'd like to get consensus around, and I probably need to beef up the testing before really issuing it as a PR

333