Add support for textDocument/signatureHelp

[rcjsuen]

Due to the way that snippets are currently implemented in this language server, textDocument/signatureHelp isn't that useful because there is no client-neutral way of invoking textDocument/signatureHelp after a textDocument/completion request (as described by Microsoft/language-server-protocol#274).

Still, this doesn't mean that textDocument/signatureHelp shouldn't be supported. I think the initial cut of this feature should cover the instruction flags in COPY and HEALTHCHECK and the escape parser directive. We can then add support for instruction arguments after that.