Rework inlay hints to match the spec

[kddnewton]

At the time that I wrote the inlay hints stuff for the language server, inlay hints were not actually a part of the LSP. Since then, they have been added. If we conform to the spec, we can greatly simplify vscode-syntax-tree because it won't have to handle deduping itself and can just let the client handle it. Additionally, it will work for clients that aren't vscode.