Add the ":LspDiag" command and add sub-commands for the various Lsp

Add the ":LspDiag" command and add sub-commands for the various Lsp
diagnostic functionality.  Update the test scripts to use the new set of
commands.  Add support for dynamically changing the value of the
diagnostic options.  Remove the ":LspDiagHighlightEnable" and the
":LspDiagHighlightDisable" commands (these are replaced by the ":LspDiag
highlight" command)