tl: do not complain about unknown keys in tlconfig.lua

spec/config/errors.lua

@@ -1,13 +1,12 @@
 describe("config", function()
-   it("should report unknown keys in tlconfig.lua", function()
+   it("should not report unknown keys in tlconfig.lua", function()
       util.run_mock_project(finally, {
          dir_structure = {
             ["tlconfig.lua"] = [[return { foo = "hello" }]],
          },
          cmd = "check",
          args = { "tlconfig.lua" },
-         cmd_output = [[* in key "foo": unknown config key 'foo'
-========================================
+         cmd_output = [[========================================
 Type checked tlconfig.lua
 0 errors detected -- you can use:
 

tl

@@ -786,7 +786,7 @@ local function validate_config(config)
       if k == "preload_modules" then
          fail(k, "this key is no longer supported. To load a definition globally into the environment, use global_env_def.")
       elseif not valid_keys[k] then
-         warning(k, "unknown config key '" .. k .. "'")
+         -- skip invalid keys, to be used by other tools
       elseif type(valid_keys[k]) == "table" then
          if not valid_keys[k][v] then
             fail(k, "expected one of: %s", table.concat(keys(valid_keys[k][v]), ", "))