feat(lsp): implement vim.lsp.diagnostic.redraw() (#15203)

Add a new function to redraw diagnostics from the current diagnostic
cache, without receiving a "publishDiagnostics" message from the server.
This is already being done in two places in the Lua stdlib, so this
function unifies that functionality in addition to providing it to third
party plugins.

An example use case for this could be a command or key-binding for
toggling diagnostics virtual text. The virtual text configuration option
can be toggled using `vim.lsp.with` followed by
`vim.lsp.diagnostic.redraw()` to immediately redraw the diagnostics
with the updated setting.

runtime/lua/vim/lsp.lua

@@ -453,15 +453,7 @@ local function text_document_did_open_handler(bufnr, client)
 
   -- Next chance we get, we should re-do the diagnostics
   vim.schedule(function()
-    vim.lsp.handlers["textDocument/publishDiagnostics"](
-      nil,
-      "textDocument/publishDiagnostics",
-      {
-        diagnostics = vim.lsp.diagnostic.get(bufnr, client.id),
-        uri = vim.uri_from_bufnr(bufnr),
-      },
-      client.id
-    )
+    vim.lsp.diagnostic.redraw(bufnr, client.id)
   end)
 end