refactor: move vim.lsp.diagnostic to vim.diagnostic

This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).

More discussion at [1].

[1]: https://github.com/neovim/neovim/pull/15585

test/functional/plugin/lsp_spec.lua

@@ -1017,31 +1017,6 @@ describe('LSP', function()
     }
   end
 
-  it('highlight groups', function()
-    eq({
-      'LspDiagnosticsDefaultError',
-      'LspDiagnosticsDefaultHint',
-      'LspDiagnosticsDefaultInformation',
-      'LspDiagnosticsDefaultWarning',
-      'LspDiagnosticsFloatingError',
-      'LspDiagnosticsFloatingHint',
-      'LspDiagnosticsFloatingInformation',
-      'LspDiagnosticsFloatingWarning',
-      'LspDiagnosticsSignError',
-      'LspDiagnosticsSignHint',
-      'LspDiagnosticsSignInformation',
-      'LspDiagnosticsSignWarning',
-      'LspDiagnosticsUnderlineError',
-      'LspDiagnosticsUnderlineHint',
-      'LspDiagnosticsUnderlineInformation',
-      'LspDiagnosticsUnderlineWarning',
-      'LspDiagnosticsVirtualTextError',
-      'LspDiagnosticsVirtualTextHint',
-      'LspDiagnosticsVirtualTextInformation',
-      'LspDiagnosticsVirtualTextWarning',
-    }, exec_lua([[require'vim.lsp'; return vim.fn.getcompletion('Lsp', 'highlight')]]))
-  end)
-
   describe('apply_text_edits', function()
     before_each(function()
       insert(dedent([[