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_class_fold.vim

@@ -0,0 +1,70 @@
+vim9script
+# Vim9 :class command
+# VIM_TEST_SETUP let g:vimsyn_folding = 'cf'
+# VIM_TEST_SETUP setl fdc=2 fdl=99 fdm=syntax
+
+interface Interface1
+endinterface
+interface Interface2
+endinterface
+
+class Class1
+endclass
+
+export class Class2
+endclass
+
+abstract class Class3
+endclass
+
+export abstract class Class4
+endclass
+
+class Class5 extends Class1
+endclass
+
+export class Class6 extends Class1
+endclass
+
+class Class7 implements Interface1, Interface2
+endclass
+
+export class Class8 implements Interface1, Interface2
+endclass
+
+class Class9
+  def new()
+  enddef
+  def Method1(): void
+    def Nested1(): void
+      def Nested2(): void
+      enddef
+    enddef
+  enddef
+  def _Method2(): void
+  enddef
+  static def Method3(): void
+  enddef
+endclass
+
+abstract class Class10
+  abstract def Method1(): void
+  abstract def string(): string
+endclass
+
+
+# Issue: #14393
+
+interface Testable
+    def SetUp()
+    def TearDown()
+endinterface
+
+abstract class TestTemplate implements Testable
+    var failed: number
+    var passed: number
+
+    abstract def SetUp()
+    abstract def TearDown()
+endclass
+