runtime(vim): Update base syntax, match enum constructor type args

closes: #17840

Co-authored-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim9_generic_function_example_enum.vim

@@ -0,0 +1,36 @@
+vim9script
+# VIM_TEST_SETUP highlight link vim9DefTypeParam Todo
+# VIM_TEST_SETUP highlight link vim9EnumValue Identifier
+# VIM_TEST_SETUP let g:vimsyn_folding = "ef"
+# VIM_TEST_SETUP setl fdc=2 fdl=99 fdm=syntax
+# See: https://github.com/vim/vim/pull/17313#issuecomment-3033537127 (Aliaksei Budavei)
+
+
+enum CommonPair
+    HelloWorld<string, string>('hello', 'world'),
+    Booleans<bool, bool>(true, false)
+
+    const _fst: any
+    const _snd: any
+
+    def new<T, U>(fst: T, snd: U)
+        this._fst = fst
+        this._snd = snd
+    enddef
+
+    def First<T>(): T
+        return this._fst
+    enddef
+
+    def Second<T>(): T
+        return this._snd
+    enddef
+
+    def string(): string
+        return printf("(%s, %s)", this._fst, this._snd)
+    enddef
+endenum
+
+echo CommonPair.HelloWorld
+echo CommonPair.Booleans
+