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_interface_fold.vim

@@ -0,0 +1,36 @@
+vim9script
+# Vim :interface command
+# VIM_TEST_SETUP let g:vimsyn_folding = 'i'
+# VIM_TEST_SETUP setl fdc=2 fdl=99 fdm=syntax
+
+interface Interface1
+endinterface
+
+export interface Interface2
+endinterface
+
+interface Interface2
+  # comment
+  var var1: number
+endinterface
+
+interface Interface3
+  # comment
+  def Meth1(): number
+endinterface
+
+interface Interface4
+  # comment
+  var var1: number
+  # comment
+  def Meth1(): number
+endinterface
+
+interface Interface5
+  # comment
+  var var1: number
+  var var2: number
+  # comment
+  def Meth1(): number
+  def Meth2(): number
+endinterface