Fix highlight errors when lsp crash or stop (nvim-lua#864)

* Fix highlight errors when lsp crash or stop

It adds a check wether the client is still available before
highlighting.

If the client is not there anymore it returns `true` to unregister the
autocommand

This fix the
`method textDocument/documentHighlight is not supported by any of the servers registered for the current buffer`
errors when doing a LspRestart or the server crashes

* Delete the highlight autocommands in the LspDetatch event

* Only delete autocmds for the current buffer with the group name

* Simplify clearing the autocommands

---------

Co-authored-by: Francis Belanger <francis.belanger@ubisoft.com>

init.lua

@@ -518,13 +518,16 @@ require('lazy').setup({
           -- When you move your cursor, the highlights will be cleared (the second autocommand).
           local client = vim.lsp.get_client_by_id(event.data.client_id)
           if client and client.server_capabilities.documentHighlightProvider then
+            local highlight_augroup = vim.api.nvim_create_augroup('kickstart-lsp-highlight', { clear = true })
             vim.api.nvim_create_autocmd({ 'CursorHold', 'CursorHoldI' }, {
               buffer = event.buf,
+              group = highlight_augroup,
               callback = vim.lsp.buf.document_highlight,
             })
 
             vim.api.nvim_create_autocmd({ 'CursorMoved', 'CursorMovedI' }, {
               buffer = event.buf,
+              group = highlight_augroup,
               callback = vim.lsp.buf.clear_references,
             })
           end
@@ -555,6 +558,14 @@ require('lazy').setup({
         end,
       })
 
+      vim.api.nvim_create_autocmd('LspDetach', {
+        group = vim.api.nvim_create_augroup('kickstart-lsp-detach', { clear = true }),
+        callback = function(event)
+          vim.lsp.buf.clear_references()
+          vim.api.nvim_clear_autocmds { group = 'kickstart-lsp-highlight', buffer = event.buf }
+        end,
+      })
+
       -- LSP servers and clients are able to communicate to each other what features they support.
       --  By default, Neovim doesn't support everything that is in the LSP specification.
       --  When you add nvim-cmp, luasnip, etc. Neovim now has *more* capabilities.