Add middleware to intercept textDocument/publishDiagnostics

[rcjsuen]

It is currently not possible for a client to inspect the contents of the textDocument/publishDiagnostics notifications from the server before letting VS Code handle them. This pull request introduces a middleware function to allow clients to intercept the contents of the notification before passing them off to VS Code.

[dbaeumer]

@rcjsuen thanks for the PR. There is one thing I am not sure of how we should do it: the middleware API currently uses VS Code API types only. With this PR it will surface LSP types. Although this allows to intercept the notification as early as possible I would nevertheless change the types to VS Code API types. In the implementation that means we first convert the LSP data into VS Code data and then call the middleware.

[rcjsuen]

@dbaeumer Hi Dirk, thanks for the review.

Should I still use NextSignature then? VS Code's Diagnostic does not contain information URI information so I am forced to change...

publishDiagnostics?: NextSignature<PublishDiagnosticsParams, void>;

...to...

publishDiagnostics?: NextSignature<Uri, VDiagnostic[], void>;

But this won't compile because NextSignature only takes two parameters.

Or should I instead write a new function like the following and just introduce a new HandleDiagnosticsSignature interface type?

handleDiagnostics?: (this: void, uri: Uri, diagnostic: VDiagnostics[], next: HandleDiagnosticsSignature) => void;

[dbaeumer]

@rcjsuen I would go with the HandleDiagnosticsSignature approach.

[rcjsuen]

@dbaeumer Thank you. I've force pushed a new change to my branch. Feel free to take a look whenever you have the time. I'm sure you're a busy man.

[dbaeumer]

Thanks for the PR.