feat(eval): exists() function supports checking v:lua functions (#26485)

Problem:  Vimscript function exists() can't check v:lua functions.
Solution: Add support for v:lua functions to exists().

runtime/doc/news.txt

@@ -339,6 +339,8 @@ The following changes to existing APIs or features add new behavior.
 
 • |vim.wait()| is no longer allowed to be called in |api-fast|.
 
+• Vimscript function |exists()| supports checking |v:lua| functions.
+
 ==============================================================================
 REMOVED FEATURES                                                 *news-removed*
 

src/nvim/eval/funcs.c

@@ -1716,6 +1716,8 @@ static void f_exists(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     } else {
       n = au_exists(p + 1);
     }
+  } else if (strncmp(p, "v:lua.", 6) == 0 && nlua_func_exists(p + 6)) {
+    n = true;
   } else {  // Internal variable.
     n = var_exists(p);
   }

src/nvim/lua/executor.c

@@ -2313,3 +2313,21 @@ void nlua_init_defaults(void)
     os_errmsg("\n");
   }
 }
+
+/// check lua function exist
+bool nlua_func_exists(const char *lua_funcname)
+{
+  MAXSIZE_TEMP_ARRAY(args, 1);
+  size_t length = strlen(lua_funcname) + 8;
+  char *str = xmalloc(length);
+  vim_snprintf(str, length, "return %s", lua_funcname);
+  ADD_C(args, CSTR_AS_OBJ(str));
+  Error err = ERROR_INIT;
+  Object result = NLUA_EXEC_STATIC("return type(loadstring(...)()) =='function'", args, &err);
+  xfree(str);
+
+  if (result.type != kObjectTypeBoolean) {
+    return false;
+  }
+  return result.data.boolean;
+}

test/functional/lua/vim_spec.lua

@@ -3352,4 +3352,31 @@ describe('vim.keymap', function()
     eq(1, exec_lua[[return GlobalCount]])
   end)
 
+  it('exists() can check a lua function', function()
+    eq(true, exec_lua[[
+      _G.test = function() print("hello") end
+      return vim.fn.exists('v:lua.test') == 1
+    ]])
+
+    eq(true, exec_lua[[
+      return vim.fn.exists('v:lua.require("mpack").decode') == 1
+    ]])
+
+    eq(true, exec_lua[[
+      return vim.fn.exists("v:lua.require('vim.lsp').start") == 1
+    ]])
+
+    eq(true, exec_lua[[
+      return vim.fn.exists('v:lua.require"vim.lsp".start') == 1
+    ]])
+
+    eq(true, exec_lua[[
+      return vim.fn.exists("v:lua.require'vim.lsp'.start") == 1
+    ]])
+
+    eq(false, exec_lua[[
+      return vim.fn.exists("v:lua.require'vim.lsp'.unknown") == 1
+    ]])
+  end)
+
 end)