ClientCapabilities should inform ServerCapabilities

[kjeremy]

The LSP specifies that the client should first send an initialize request which contains the client capabilities. At that point the server capabilities should be sent back.

It turns out that the server capabilities can be informed by the client capabilities. For instance the client capability textDocument.codeAction.codeActionLiteralSupport informs the server whether the server capability codeActionProvider should be the boolean true or CodeActionOptions.

This is not obvious in the vscode-languageclient-node interfaces but appears to be how the built-in servers in vscode behave.

Example: https://github.com/microsoft/vscode/blob/1476c4e20b385059ca63c871257f56c6d387ad03/extensions/css-language-features/server/src/cssServerMain.ts#L128

While looking at #144