Render educational notes from the compiler

[daveyc123]

The Swift compiler exposes educational notes for certain diagnostics. These notes are markdown files that describe the diagnostic along with examples.

For example, the following code:

protocol P {}

/// A description
struct S {
  let p: some P
}

Results in a diagnostic Property declares an opaque return type, but has no initializer expression from which to infer an underlying type which includes an educational note detailed https://github.com/swiftlang/swift/blob/main/userdocs/diagnostics/opaque-type-inference.md. These markdown files are included in the toolchain under usr/share/doc/swift/diagnostics

Sourcekit-lsp supports sending the path to the markdown file along with the diagnostic, but the VSCode behavior is to open the educational note for editing.

Instead, it should show it as rendered markdown.