Add API proposal for reading files in data transfer (#148596)

* Add experimental support for reading files in data transfer

Adds a new `DataTransfer.asFile` method which lets you get file objects from a `DataTransfer`. This is currently only hooked up for drop into editors.

A few follow ups:

- Right now the file data is also read eagerly when it is transfered to the extension host. Before shipping this we would make this happen lazily instead
- The drop into editor api does not provide a nice way to do anything with the dropped files.

    We should at least support returning a `WorkspaceEdit`. However `WorkspaceEdit` only supports text files, so we would also need to add an API that lets it deal with binary files

* Make `asFile` return a value instead of a promise

`asFile().data()` already returns a promise so `asFile` doesn't also need to be async

* Trying resolving data files transfer lazily

* Cleaning up code for lazy drop

* Remove testing code

* Remove unneeded buffer serialize

* 💄

extensions/markdown-language-features/tsconfig.json

@@ -7,5 +7,6 @@
 		"src/**/*",
 		"../../src/vscode-dts/vscode.d.ts",
 		"../../src/vscode-dts/vscode.proposed.textEditorDrop.d.ts",
+		"../../src/vscode-dts/vscode.proposed.dataTransferFiles.d.ts"
 	]
 }