Add textDocument/playgrounds request

[award999]

Description

As an initial step to address swiftlang/vscode-swift#1782 in the Swift extension. Want to introduce a textDocument/playgrounds request that will follow a similar pattern to textDocument/test request where we simply syntactically parse what is likely a #Playground macro expansion for a given file and return the list.

[award999]

I have an incoming PR for this