perf(lsp): avoid textDocument/definition requests during tag completion (#37260)

Problem:
vim.lsp.tagfunc looks for the presence of 'c' (cursor) flag and issues
sync textDocument/definition requests to all clients, otherwise
workspace/symbol requests. But 'c' flag can also be set during the
insert mode completion, e.g. with an empty tag completion query, the tag
func receives pattern of '\<\k\k' with flags 'cir'.

Solution:
check for 'i' (insert mode completion) flag and don't issue any LSP
requests, return vim.NIL for immediate fallback to tags.

test/functional/plugin/lsp_spec.lua

@@ -5680,6 +5680,14 @@ describe('LSP', function()
         },
       }, result)
     end)
+
+    it('with flags including i, returns NIL', function()
+      exec_lua(function()
+        local result = vim.lsp.tagfunc('foobar', 'cir')
+        assert(result == vim.NIL, 'should not issue LSP requests')
+        return {}
+      end)
+    end)
   end)
 
   describe('cmd', function()