runtime(vim): Update base-syntax file, improve class, enum and interface highlighting

- Enable folding of class, enum and interface declarations.
- Highlight constructor names with the Function highlight group, like
  other special methods.
- Mark function definitions using special method names as errors.
- Highlight :type arguments.

fixes: #14393#issuecomment-2042796198.
closes: #13810

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim9_ex_enum_fold.vim

@@ -0,0 +1,39 @@
+vim9script
+# Vim :enum command
+# VIM_TEST_SETUP let g:vimsyn_folding = 'ef'
+# VIM_TEST_SETUP setl fdc=2 fdl=99 fdm=syntax
+
+interface Interface1
+endinterface
+interface Interface2
+endinterface
+
+enum Enum1
+endenum
+
+export enum Enum2
+endenum
+
+enum Enum3
+  Value1,
+  Value2,
+  Value3
+endenum
+
+enum Enum4
+  Value1,
+  Value2,
+  Value3
+  def Method1()
+  enddef
+endenum
+
+enum Enum5 implements Interface1, Interface2
+    Value1,
+    Value2,
+    Value3
+    def Method1()
+      def Nested()
+      enddef
+    enddef
+endenum